I suggest you use a tabularx environment and the X column type (both provided by the tabularx package) for the first and fourth columns.
The tabularx environment takes as one of its arguments the desired width of the environment; I would choose \textwidth, i.e., let the table occupy the full width of the textblock. The X column type (and the derived type L used in the example below) allow for automatic line breaks, while relieving the writer (you!) of the chore of having to compute the exact column widths.

\documentclass{article}
\usepackage{adjustbox}
\usepackage{tabularx,ragged2e,booktabs}
\newcolumntype{L}{>{\RaggedRight\arraybackslash}X}
\begin{document}
%%%\begin{center}
\begin{table}[h!]
\begin{adjustbox}{width=1.0\textwidth}
\begin{tabular}{|llll|}
\hline
Item & Manufacturer & Catalogue number & Use/Purpose \\
\hline \hline
DMEM Dulbecco's Modified Eagle Medium & Life Technologies & 41965-039
& Maintain cultured RBL-2H3 cells \\
\hline
Penicillin-streptomycin
& Life Technologies & 15140-122 & Supplement for cell culture medium \\
\hline \hline
\end{tabular}
\end{adjustbox}
\caption{before: tabular, ``l'' columns, adjustbox} \label{tab1}
\end{table}
%%%\end{center}
\begin{table}[h!]
\begin{tabularx}{\textwidth}{@{}LllL@{}}
\toprule
Item & Manufacturer & Catalogue & Use/Purpose \\
& & Number & \\
\midrule
DMEM Dulbecco's Modified Eagle Medium & Life Technologies & 41965-039 & Maintain cultured RBL-2H3 cells \\
\addlinespace
Penicillin-streptomycin
& Life Technologies & 15140-122 & Supplement for cell culture medium \\
\bottomrule
\end{tabularx}
\caption{after: tabularx, ``L'' columns, booktabs-based lines}\label{tab2}
\end{table}
\end{document}
pcolumn type? – egreg May 19 '15 at 21:14