Four side by side figures each pair have one common caption

Question

I am using the following LaTeX code to put four figures in two rows with two figures side by side so that both of them are on a line and have a one common caption. Unfortunately when I am using the caption the LaTeX file does not execute properly. How can I do the above procedure? There is other code that does not work. I am using MiKTeX 2.8 and TeXnicCenter stable 2.3.

Here is my code:

\begin{figure}[htp]

\begin{tabular}{cc}

% Requires \usepackage{graphicx}
\includegraphics[width=13cm]{fig1.eps}&

\hspace{-5cm} \includegraphics[width=13cm]{fig2}\caption{}\

\includegraphics[width=13cm]{fig3.eps}& \hspace{-5cm} \includegraphics[width=13cm]{fig4.eps}\caption{}\
\end{tabular} \end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
SECOND CODE IS
\begin{figure}
\centering
\begin{tabular}{cc}
\includegraphics{fig1} &
\includegraphics{fig2}
\end{tabular}
\caption{.....}
\end{figure}
\begin{figure}
\centering
\begin{tabular}{cc}
\includegraphics{fig3} &
\includegraphics{fig4}
\end{tabular}
\caption{.....}
\end{figure}

Answer 1

One option:

\documentclass{article}
\usepackage[demo]{graphicx}

\begin{document}

\begin{figure}
  \centering
  \includegraphics[width=.4\linewidth]{fig2}\quad
  \includegraphics[width=.4\linewidth]{fig3}\par
  \caption{the caption for the first group}\par\bigskip
  \label{fig:testa}
  \includegraphics[width=.4\linewidth]{fig4}\quad
  \includegraphics[width=.4\linewidth]{fig4}
  \caption{the caption for the second group}
  \label{fig:testb}
\end{figure}

\end{document}

Your settings for the width (13cm) were perhaps too wide for a standard document; it's better to control the width using a fraction of \linewidth, as I did in my example code.

The demo option for graphicx simply replaces actual figures with black rectangles; do not use that option in your actual document.