Positioning two figures (with subfigures) which use 1.5 rows each to occupy 3 rows instead of 4

Question

I have two figures with 4 subfigures each. They are currently placed like this

A1 A2 A3
   A4
- Ac ---
B1 B2 B3
   B4
- Bc ---

Ac and Bc denote captions. Each subfigure also has its own caption.

What's the most space efficient way to organize this situation?

The best I could come up with is the following but I'm not sure how to achieve it

A1 A2 A3
A4    B4
-Ac--
B1 B2 B3
-Bc-----

Latex code to generate this looks similar to

\begin{figure}
  \begin{subfigure}{0.33\textwidth} A1\end{subfigure}
  \begin{subfigure}{0.33\textwidth} A2\end{subfigure}
  \begin{subfigure}{0.33\textwidth} A3\end{subfigure}
  \begin{subfigure}{0.33\textwidth} A4\end{subfigure}
\end{figure}
\begin{figure}
  \begin{subfigure}{0.33\textwidth} B1\end{subfigure}
  \begin{subfigure}{0.33\textwidth} B2\end{subfigure}
  \begin{subfigure}{0.33\textwidth} B3\end{subfigure}
  \begin{subfigure}{0.33\textwidth} B4\end{subfigure}
\end{figure}

This is in a single-column format (LNCS) if it matters.

Answer 1

If you have very short captions you could do something like this:

\documentclass{article}
\usepackage{graphicx}
\usepackage[list=true]{subcaption}% add to LOF
\usepackage{duckuments}

\renewcommand{\thesubfigure}{\thefigure\alph{subfigure}}% affects LOF and \ref too
\captionsetup[subfigure]{labelformat=simple}

\begin{document}
\listoffigures

\begin{figure}
  \setlength{\dimen0}{\dimexpr \textwidth-2\columnsep}%
  \divide\dimen0 by 3
  \sbox0{\begin{subfigure}{\dimen0}% measure height
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}}%
  \usebox0\hfill
  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}\hfill
  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}

  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}\hfill
  \begin{minipage}[c][\dimexpr \ht0+\dp0][s]{\dimen0}
    \setlength{\abovecaptionskip}{0pt}%
    \setlength{\belowcaptionskip}{0pt}%
    \caption{Top caption}
    \vfill
    \caption{Bottom caption}
  \end{minipage}\hfill
  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}

  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}\hfill
  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}\hfill
  \begin{subfigure}{\dimen0}
    \includegraphics[width=\textwidth]{example-image-duck}
    \caption{}
  \end{subfigure}
\end{figure}
\end{document}