from your question, subfigure
width roughly equals half of \textwidth
and image width equals half of subfigure
width. So, you can put all four sub-images in one row (first figure), or maybe in two rows (second figure).
Latex code for both cases:
%%%%%%%% first image
\begin{figure}
\begin{subfigure}[b]{0.24\textwidth} % width of upper left subfigure
\includegraphics[width=\linewidth]{Test2/22_macro.png}
\caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of upper right subfigure
\centering
\includegraphics[width=\linewidth]{Test2/23_macro.png}
\caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of lower left subfigure
\includegraphics[width=\linewidth]{Test2/26_macro.png}
\caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.24\textwidth} % width of lower right subfigure
\includegraphics[width=\linewidth]{Test2/27_macro.png}
\caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
\caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}
%%%%%%%% second image
\begin{figure}
\begin{subfigure}[b]{0.49\textwidth} % width of upper left subfigure
\includegraphics[width=\linewidth]{Test2/22_macro.png}
\caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.49\textwidth} % width of upper right subfigure
\centering
\includegraphics[width=\linewidth]{Test2/23_macro.png}
\caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}
\begin{subfigure}[b]{0.49\textwidth} % width of lower left subfigure
\includegraphics[width=\linewidth]{Test2/26_macro.png}
\caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\hfill
\begin{subfigure}[b]{0.49\textwidth} % width of lower right subfigure
\includegraphics[width=\linewidth]{Test2/27_macro.png}
\caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
\caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}
%%%%%%%% third image
\begin{figure}
\centering
\begin{subfigure}[b]{0.25\textwidth} % width of upper left subfigure
\includegraphics[width=\linewidth]{Test2/22_macro.png}
\caption{Sample 2.1 - \SI{20}{ppm}} % subcaption
\end{subfigure}%
\quad
\begin{subfigure}[b]{0.25\textwidth} % width of upper right subfigure
\centering
\includegraphics[width=\linewidth]{Test2/23_macro.png}
\caption{Sample 2.3 - \SI{50}{ppm}} % subcaption
\end{subfigure}
\begin{subfigure}[b]{0.25\textwidth} % width of lower left subfigure
\includegraphics[width=\linewidth]{Test2/26_macro.png}
\caption{Sample 2.6 - \SI{100}{ppm}} % subcaption
\end{subfigure}%
\quad
\begin{subfigure}[b]{0.25\textwidth} % width of lower right subfigure
\includegraphics[width=\linewidth]{Test2/27_macro.png}
\caption{Samples 2.7 - \SI{150}{ppm}} % subcaption
\end{subfigure}%
\caption{common caption} % caption for whole figure
\label{fig:foursubfig}
\end{figure}

Edit:
You can also have small images in two rows (third figure).

subfigures
whose width is about half the\textwidth
. Then you fill them with images whose width is half the one of thesubfigure
, hence the blank space. – BambOo Oct 26 '18 at 13:56