I am arranging four images as follows.
\documentclass{article}
\usepackage[demo]{graphicx}
\usepackage{subcaption, tabu}
\begin{document}
\begin{figure}[t]
\begin{tabular}{c|c} %vertical line I added | between cc
\subcaptionbox{}{\includegraphics[width = 0.45\columnwidth]{a}}&
\subcaptionbox{}{\includegraphics[width = 0.45\columnwidth]{c}}\\
\hline
\subcaptionbox{}{\includegraphics[width = 0.45\columnwidth]{b}}&
\subcaptionbox{}{\includegraphics[width = 0.45\columnwidth]{d}}
\end{tabular}
\caption{Overall caption}
\label{fig:xxx}
\end{figure}
\end{document}
I would like to move the subcaptions (that is (a), (b), (c), (d)) closer to the images and potentially even onto them as the images are mostly white.
The current answer at Subfigures with number inside figure doesn't seem to move the caption vertically sadly. The answer to my question looks like it might be How can I modify vertical space between figure and caption? but \setlength{\belowcaptionskip}{0pt} has no effect for me.
What is the right way to do this?
