[Tex/LaTex] How to layout four figures with floatrow, two of them vertically

Question

how can I get a layout like the one below in my document?

_____________  _____________  _______________
|Big Picture1| |Big Picture2| |Small Picture1|
|            | |            | |______________|
|            | |            |   c) subfigure
|            | |            | _______________
|            | |            | |Small Picture2|
|____________| |____________| |______________|
 a) subfigure   b) subfigure    d) subfigure
            Fig. 1: four subfigures

I tried the floatrow package and managed to get two figures with two subfigures, respectively. But how do I combine them into one figure?

\begin{figure}
\ffigbox{
\begin{subfloatrow}[2]
\ffigbox[\FBwidth][][]{\caption{subcaption1}}
    {\includegraphics[scale=0.4]{big_picture1}}
\ffigbox[\FBwidth][][]{\caption{subcaption2}}
    {\includegraphics[scale=0.4]{big_picture2}}
\end{subfloatrow}
}{\caption{figure1}\label{label1}}
\end{figure}

\begin{figure}
\ffigbox{
\begin{subfloatrow}[1]
    \ffigbox[\FBwidth][][]{\caption{subcaption3}}
        {\includegraphics[scale=0.4]{small_picture1}}
\end{subfloatrow} \\
\begin{subfloatrow}[1]
    \ffigbox[\FBwidth][][]{\caption{subcaption4}}
        {\includegraphics[scale=0.4]{small_picture2}}
\end{subfloatrow}
}{\caption{figure2}\label{label2}}
\end{figure}

Answer 1

Here's one possibility; you might need to adjust some lengths according to your actual needs:

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

\begin{document}

\begin{figure}
\ffigbox[][7.8cm]{%
\begin{subfloatrow}
  \ffigbox[\FBwidth][]
    {\caption{subfigure-a}}
    {\includegraphics[width=3cm,height=6cm]{bigpicture1}}
  \ffigbox[\FBwidth][]
    {\caption{subfigure-b}}
    {\includegraphics[width=3cm,height=6cm]{bigpicture2}}
\end{subfloatrow}\hspace*{\columnsep}%
\begin{subfloatrow}
  \hsize0.7\hsize
  \vbox to 6.3cm{%
  \ffigbox[\FBwidth]
    {\caption{subfigure-c}}
    {\includegraphics[width=3cm,height=2cm]{smallpicture1}}\vss
  \ffigbox[\FBwidth]
    {\caption{subfigure-d}}
    {\includegraphics[width=3cm,height=2cm]{smallpicture2}}
  }%
\end{subfloatrow}
}{\caption{four subfigures}}
\end{figure}

\end{document}