How to print graphics on a new line in floatrow/subfloatrow

Question

How can I skip to a newline in a subfloatrow environment (floatrow package) after two graphics. I want to typeset two graphics A.pdf and B.pdf on the same line, C.pdf and D.pdf should be on a new line. All graphics should be part of the same subfloatrow, considering the labels and captions. I'm missing part of the floatrow concept so I tried silly things like introducing newlines (\\ and \newline) which of course won't work.

This is the code I used:

preamble:

\documentclass{article}
\usepackage{floatrow}
\usepackage{graphicx}
\usepackage{subfig}
\usepackage{caption}

\begin{document}  
    \clearpage
    \begin{figure}[h!]
    \ffigbox[\FBwidth]
    {\begin{subfloatrow}
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{A.pdf}}%
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{B.pdf}}%
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{C.pdf}}%
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{D.pdf}}%
        \end{subfloatrow}}
    {\caption{this is a caption}}
    \end{figure}
\end{document}

Answer 1

For each row you need to use the subfloatrow environment:

\documentclass{article}
\usepackage{floatrow}
\usepackage[demo]{graphicx}
\usepackage{subfig}
\usepackage{caption}

\begin{document}  

\begin{figure}[h!]
  \ffigbox[\FBwidth]
    {\begin{subfloatrow}
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{A.pdf}}%
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{B.pdf}}%
      \end{subfloatrow}
      \begin{subfloatrow}
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{C.pdf}}%
        \sidesubfloat[]{\includegraphics[width = 0.4\textwidth]{D.pdf}}%
      \end{subfloatrow}}
    {\caption{this is a caption}}
\end{figure}

\end{document}

I used the demo option for graphicx to make my example code compilable for everyone; do not use that option in your actual code.