It is not clear if you want to justify increasing inter-word spacing, or also the letter spacing, with or without hyphenation, and in multiline cells, if the last line must be also justified.
The table shows possible combinations playing with \makebox in cell with single lines and \parfillskip in cells with paragraphs, and \so (of soul package) to increase the space between characters, in already justified column and left aligned columns.

The bad spacing between words forcing justification could be mitigated in some extent playing with \emergencystretch, or using themicrotype package Not showed in the MWE), but do not expect miracles. And including spaces between characters only gets worse.
So, whatever the true goal is, the conclusion IMHO is the same: Do not do that! Only in very wide columns, with a lot of text, full justification will be not harmful, and I will not suggest you make tables with a lot of text, but the opposite.
The MWE code:
\documentclass{article}
\usepackage[margin=1cm]{geometry}
\usepackage{array}
\usepackage{ragged2e}
\usepackage{booktabs} % good for any table ... except for this MWE ;-)
\extrarowheight2pt
\usepackage{soul}
\begin{document}
\begin{tabular}{|l|l|>{\raggedright}p{6cm}|p{6cm}|}\hline
1 & % reference row (normal text)
Some content. &
Some content, some content some content. &
Some content, some content some content. \\\hline
2 & % interword spacing
\parfillskip0pt Some content. & % ooopss !!
\parfillskip0pt Some content, some content some content. & % ooopss !!
\parfillskip0pt Some content, some content some content. \\\hline
3 & % interword spacing again
\makebox[4cm][s]{Some content.} &
\justifying\parfillskip0pt\noindent Some content, some content some content. &
\parfillskip0pt Some content, some content some content. \\\hline
4 & % interletter spacing
\so{Some content.} &
\so{Some content, some content some content.} &
\so{Some content, some content some content.} \\\hline
5 & % interword and interletter spacing
\makebox[4cm][s]{\so{Some content.}} &
\justifying\parfillskip0pt\noindent \parfillskip0pt \so{Some content, some content some content.} &
\parfillskip0pt \so{Some content, some content some content.} \\\hline
\end{tabular}
\end{document}