How to solve text wrapping problem in longtable latex?

Question

    \documentclass[12pt]{article}
    \usepackage{longtable}
    \usepackage[landscape]{geometry}
    \usepackage{pdflscape}

    \begin{document}

    \begin{center}

    \begin{longtable}{|p{3cm}|p{6cm}|p{6cm}|p{6cm}|}
    \caption{A simple longtable example}\\
    \hline
    \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
    \hline
    \endfirsthead
    \multicolumn{4}{c}%
    {\tablename\ \thetable\ -- \textit{Continued from previous page}} \\
    \hline
    \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
    \hline
    \endhead
    \hline \multicolumn{4}{r}{\textit{Continued on next page}} \\
    \endfoot
    \hline
    \endlastfoot
    \begin{itemize} \item TEAM/ UNIT/ Organisation  \end{itemize}  
   & \begin{itemize}
    \item To maintain staff passion to maintain data provider trust in 
    releasing data because the data providers could get feedbacks during 
    verifications.
   \end{itemize}
   & \begin{enumerate}\item To explore the Idea of a merger* with  existing 
   expertise/resources in MOH to upgrade the hardcopy reports into 
   electronic version ie bona fide Performance Dashboard possibly using the 
   system that has been developed. 
   \item Use the prototype Performance Dashboard to help clients “see” its 
   potentials to help them in their work, so that they can come up with what 
   matters to them (direction) that eventually helps MHPU/PIK to revise 
   their strategic plans/performance metrics
   \end{enumerate}
  & \begin{itemize} \item To maintain staff passion to maintain data 
   provider trust in releasing data because the data providers could get 
   feedbacks during verifications. 
  \end{itemize}  \\
  \hline
  Team/ Unit/ Organisation & To maintain staff passion to maintain data 
  provider trust in releasing data because the data providers could get 
  feedbacks during verifications.   

   & 1.To maintain staff passion to maintain data provider trust in 
   releasing data because the data providers could get feedbacks during 
   verifications.

   2.To explore the Idea of a merger* with  existing expertise/resources to 
   upgrade the hardcopy reports into electronic version ie  bona fide 
   Performance Dashboard possibly using the system that has been developed 
   by 

   3. Use the prototype Performance Dashboard to help clients “see” its 
   potentials to help them in their work, so that they can come up with
   what matters to them (direction) that eventually helps MHPU/PIK to revise 
   their strategic plans/performance metrics.       

   4. To explore the Idea of a merger* with  existing expertise/resources in    
   MOH to upgrade the hardcopy reports into electronic version ie bona fide 
   Performance Dashboard possibly using the system that has been developed. 

   5. To explore the Idea of a merger* with  existing expertise/resources 
   to upgrade the hardcopy reports into electronic version ie bona fide 
   Performance Dashboard possibly using the system that has been developed.

    & To maintain staff passion to maintain data provider trust in releasing 
    data because the data providers could get feedbacks during 
    verifications. \\

    11111111111 & 22222222222222222222222 & 3333333333333333 & 444444 \\ 
    1 & 2 & 3 & 4 \\ 1 & 2 & 3 & 4 \\
    1 & 2 & 3 & 4 \\ 1 & 2 & 3 & 4 \\ 1 & 2 & 3 & 4 \\
    1 & 2 & 3 & 4 \\1 & 2 & 3 & 4 \\ 1 & 2 & 3 & 4 \\ 
     1 & 2 & 3 & 4 

    \end{longtable}

    \end{center}

   \end{document}

Answer 1

With the help of enumitem and column widths calculated relative to the textwidth you can achieve the following:

Using different column widths you can save quite a lot of space:

 \documentclass[12pt]{article}
 \usepackage{longtable}
 \usepackage[landscape]{geometry}
 \usepackage{ragged2e}
 \usepackage{calc,array}

\usepackage{enumitem}
\newlist{tabitem}{itemize}{1}
\newlist{tabenum}{enumerate}{1}

\setlist[tabitem]{nosep,
                  topsep= 0pt,
                  partopsep=0pt,
                  leftmargin= *,
                  label=\textbullet,
                  before=\vspace{-0.6\baselineskip},
                  after=\vspace{-\baselineskip}
                  } 
 \setlist[tabenum]{nosep,
                  topsep= 0pt,
                  partopsep=0pt,
                  leftmargin= *,
                  label=\arabic*.,
                  before=\vspace{-0.6\baselineskip},
                  after=\vspace{-\baselineskip}
                  } 

 \begin{document}
 {\RaggedRight
 \begin{longtable}{|p{0.145\textwidth-2\tabcolsep-2\arrayrulewidth}|p{0.285\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.285\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.285\textwidth-2\tabcolsep-\arrayrulewidth}|}
 \caption{A simple longtable example}\\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endfirsthead
 \multicolumn{4}{c}%
 {\tablename\ \thetable\ -- \textit{Continued from previous page}} \\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endhead
 \hline \multicolumn{4}{r}{\textit{Continued on next page}} \\
 \endfoot
 \hline
 \endlastfoot
 \begin{tabitem}    \item TEAM/ UNIT/ Organisation  \end{tabitem}  
 & \begin{tabitem}
 \item To maintain staff passion to maintain data provider trust in releasing 
 data because the data providers could get feedbacks during verifications.
 \end{tabitem}
 & \begin{tabenum}\item To explore the Idea of a merger* with  existing 
 expertise/resources in MOH to upgrade the hardcopy reports into electronic 
 version ie  bona fide Performance Dashboard possibly using the system that 
  has been developed. 
\item Use the prototype Performance Dashboard to help clients “see” its 
potentials to help them in their work, so that they can come up with what 
matters to them (direction) that eventually helps MHPU/PIK to revise their 
strategic plans/performance metrics
\end{tabenum}
&   \begin{tabitem} \item To maintain staff passion to maintain data 
provider trust in releasing data because the data providers could get 
feedbacks during verifications. 
\end{tabitem}  \\
\hline
\end{longtable}}

{\RaggedRight
 \begin{longtable}{|p{0.145\textwidth-2\tabcolsep-2\arrayrulewidth}|p{0.2\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.455\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.2\textwidth-2\tabcolsep-\arrayrulewidth}|}
 \caption{A simple longtable example}\\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endfirsthead
 \multicolumn{4}{c}%
 {\tablename\ \thetable\ -- \textit{Continued from previous page}} \\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endhead
 \hline \multicolumn{4}{r}{\textit{Continued on next page}} \\
 \endfoot
 \hline
 \endlastfoot
 \begin{tabitem}    \item TEAM/ UNIT/ Organisation  \end{tabitem}  
 & \begin{tabitem}
 \item To maintain staff passion to maintain data provider trust in releasing 
 data because the data providers could get feedbacks during verifications.
 \end{tabitem}
 & \begin{tabenum}\item To explore the Idea of a merger* with  existing 
 expertise/resources in MOH to upgrade the hardcopy reports into electronic 
 version ie  bona fide Performance Dashboard possibly using the system that 
  has been developed. 
\item Use the prototype Performance Dashboard to help clients “see” its 
potentials to help them in their work, so that they can come up with what 
matters to them (direction) that eventually helps MHPU/PIK to revise their 
strategic plans/performance metrics
\end{tabenum}
&   \begin{tabitem} \item To maintain staff passion to maintain data 
provider trust in releasing data because the data providers could get 
feedbacks during verifications. 
\end{tabitem}  \\
\hline
\end{longtable}}
\end{document}

---

Regarding the edited question:

longtable can only break a table between rows. If you have a long list in a single table row there can not be a pagebreak within this list as you see in your example. To enable page breaks, you can split the list into several rows and use the series and resume approach for continued numbering as shown in the following example:

\documentclass[12pt]{article}
 \usepackage{longtable}
 \usepackage[landscape]{geometry}
 \usepackage{ragged2e}
 \usepackage{calc,array}

\usepackage{enumitem}
\newlist{tabitem}{itemize}{1}
\newlist{tabenum}{enumerate}{1}

\setlist[tabitem]{nosep,
                  topsep= 0pt,
                  partopsep=0pt,
                  leftmargin= *,
                  label=\textbullet,
                  before=\vspace{-0.6\baselineskip},
                  after=\vspace{-\baselineskip}
                  } 
 \setlist[tabenum]{nosep,
                  topsep= 0pt,
                  partopsep=0pt,
                  leftmargin= *,
                  label=\arabic*.,
                  before=\vspace{-0.6\baselineskip},
                  after=\vspace{-\baselineskip},
                  } 

 \begin{document}

{\RaggedRight
 \begin{longtable}{|p{0.145\textwidth-2\tabcolsep-2\arrayrulewidth}|p{0.2\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.455\textwidth-2\tabcolsep-\arrayrulewidth}|p{0.2\textwidth-2\tabcolsep-\arrayrulewidth}|}
 \caption{A simple longtable example}\\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endfirsthead
 \multicolumn{4}{c}%
 {\tablename\ \thetable\ -- \textit{Continued from previous page}} \\
 \hline
 \textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
 \hline
 \endhead
 \hline \multicolumn{4}{r}{\textit{Continued on next page}} \\
 \endfoot
 \hline
 \endlastfoot
 \begin{tabitem}    \item TEAM/ UNIT/ Organisation  \end{tabitem}  
 & \begin{tabitem}
 \item To maintain staff passion to maintain data provider trust in releasing 
 data because the data providers could get feedbacks during verifications.
 \end{tabitem}
 & \begin{tabenum}[series=enum]\item To explore the Idea of a merger* with  existing 
 expertise/resources in MOH to upgrade the hardcopy reports into electronic 
 version ie  bona fide Performance Dashboard possibly using the system that 
  has been developed. 
\item Use the prototype Performance Dashboard to help clients “see” its 
potentials to help them in their work, so that they can come up with what 
matters to them (direction) that eventually helps MHPU/PIK to revise their 
strategic plans/performance metrics
\end{tabenum}
&   \begin{tabitem} \item To maintain staff passion to maintain data 
provider trust in releasing data because the data providers could get 
feedbacks during verifications. 
\end{tabitem}  \\
&& \begin{tabenum}[resume=enum] \item To explore the Idea of a merger* with  existing 
 expertise/resources in MOH to upgrade the hardcopy reports into electronic 
 version ie  bona fide Performance Dashboard possibly using the system that 
  has been developed. 
\item Use the prototype Performance Dashboard to help clients “see” its 
potentials to help them in their work, so that they can come up with what 
matters to them (direction) that eventually helps MHPU/PIK to revise their 
strategic plans/performance metrics
\end{tabenum} & \\
&& \begin{tabenum}[resume=enum] \item To explore the Idea of a merger* with  existing 
 expertise/resources in MOH to upgrade the hardcopy reports into electronic 
 version ie  bona fide Performance Dashboard possibly using the system that 
  has been developed. 
\item Use the prototype Performance Dashboard to help clients “see” its 
potentials to help them in their work, so that they can come up with what 
matters to them (direction) that eventually helps MHPU/PIK to revise their 
strategic plans/performance metrics
\end{tabenum} & \\
\hline
\end{longtable}}
\end{document}

Answer 2

As you've already been informed, the longtable environment can only create page breaks at the end of a "row". However, by making the second data column about four times as wide as the other two, it's possible to fit both "rows" of the longtable on a single page.

In the following code, I've also customized the itemize and enumerate environment, to make them take up as little space as possible.

Since the columns are quite narrow, I suggest you switch from fully-justified to raggedright appearance.

Another hint: you seem to use the symbol / ("slash") quite a bit to form word pairs and even triples. LaTeX will in general not allow line breaks at the / symbols. I suggest you replace all instances of / with \slash (note the extra space); that way, line breaks are possible once again. For example, I suggest you switch from Team/Unit/Organisation to Team\slash Unit\slash Organisation.

\documentclass[12pt,a4paper]{article}
\usepackage{longtable}
\usepackage[margin=2cm,landscape]{geometry}
\usepackage[english]{babel}

\usepackage{array,ragged2e}
\newcolumntype{P}[1]{>{\RaggedRight\arraybackslash}p{#1}}

\usepackage{enumitem}
%% customize 'itemize' and 'enumerate' environments:
\setlist[itemize]{wide=0pt,nosep,
    before = \vspace{-\baselineskip},
    after  = \vspace{-\baselineskip}}
\setlist[enumerate]{wide=0pt,nosep,
    before = \vspace{-\baselineskip},
    after  = \vspace{-\baselineskip}}


\begin{document}
\setlength\extrarowheight{1.5pt}

\begin{longtable}{|P{2.5cm}|P{4cm}|P{12cm}|P{4cm}|}

%% headers and footers

\caption{A single-page longtable example}\\
\hline
\textbf{Items} & \textbf{2018} & \textbf{2019} & \textbf{2020} \\
\hline
\endfirsthead

\multicolumn{4}{c}{\tablename\ \thetable,
 \textit{Continued from previous page}} \\
\hline
\textbf{Items} & \textbf{2018} & 
\textbf{2019}  & \textbf{2020} \\
\hline
\endhead

\hline 
\multicolumn{4}{r}{\textit{Continued on next page}} \\
\endfoot

\hline
\endlastfoot


%% body of table

\begin{itemize} 
\item TEAM\slash UNIT\slash Organisation  
\end{itemize}  
& 
\begin{itemize}
\item To maintain staff passion to maintain data provider trust in releasing data because the data providers could get feedbacks during verifications.
\end{itemize}
&  
\begin{enumerate}
\item To explore the Idea of a merger* with existing expertise\slash resources in MOH to upgrade the hardcopy reports into electronic version ie bona fide Performance Dashboard possibly using the system that has been developed. 
\item Use the prototype Performance Dashboard to help clients ``see'' its potentials to help them in their work, so that they can come up with what matters to them (direction) that eventually helps MHPU\slash PIK to revise their strategic plans\slash performance metrics
\end{enumerate}
& 
\begin{itemize} 
\item To maintain staff passion to maintain data provider trust in releasing data because the data providers could get feedbacks during verifications. 
\end{itemize}  
\\  % end of first "row"
\hline

Team\slash Unit\slash Organisation 
& 
To maintain staff passion to maintain data provider trust in releasing data because the data providers could get feedbacks during verifications.   
& 
\begin{enumerate}
\item To maintain staff passion to maintain data provider trust in releasing data because the data providers could get feedbacks during verifications.
\item To explore the Idea of a merger* with existing expertise\slash resources to upgrade the hardcopy reports into electronic version ie  bona fide Performance Dashboard possibly using the system that has been developed by 
\item Use the prototype Performance Dashboard to help clients ``see'' its potentials to help them in their work, so that they can come up with what matters to them (direction) that eventually helps MHPU\slash PIK to revise their strategic plans\slash performance metrics.
\item To explore the Idea of a merger* with existing expertise\slash resources in MOH to upgrade the hardcopy reports into electronic version ie bona fide Performance Dashboard possibly using the system that has been developed.
\item To explore the Idea of a merger* with existing expertise\slash resources to upgrade the hardcopy reports into electronic version ie bona fide Performance Dashboard possibly using the system that has been developed.
\end{enumerate}
& 
To maintain staff passion to maintain data provider trust in releasing data because the data providers could get feedbacks during verifications. 
\\ % end of second "row"
\end{longtable}
\end{document}