One subcaption for two subfigures

Yes, just place two images in each \subcaptionbox.

\documentclass{article}
\usepackage[demo]{graphicx} % the demo option replace images with black boxes
\usepackage{subcaption}
\begin{document}
\begin{figure}
  \centering
  \subcaptionbox{image 1 and 2}%
    {\includegraphics{image1.png}\hspace{0.5cm}\includegraphics{image2.png}}%

\bigskip

  \subcaptionbox{image 3 and 4}%
    {\includegraphics{image3.png}\hspace{0.5cm}\includegraphics{image4.png}}%
  \caption{Four images.}
\end{figure}
\end{document}