Problem on subfigure 2X2
Each of your \subfigure
s is 1\textwidth
wide - there's no way LaTeX could fit two of these in a row. Also you don't need the minipage there.
Try something like this:
\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}
\usepackage{mwe}
\begin{document}
\begin{figure*}
\centering
\begin{subfigure}[b]{0.475\textwidth}
\centering
\includegraphics[width=\textwidth]{Example-Image}
\caption[Network2]%
{{\small Network 1}}
\label{fig:mean and std of net14}
\end{subfigure}
\hfill
\begin{subfigure}[b]{0.475\textwidth}
\centering
\includegraphics[width=\textwidth]{Example-Image}
\caption[]%
{{\small Network 2}}
\label{fig:mean and std of net24}
\end{subfigure}
\vskip\baselineskip
\begin{subfigure}[b]{0.475\textwidth}
\centering
\includegraphics[width=\textwidth]{Example-Image}
\caption[]%
{{\small Network 3}}
\label{fig:mean and std of net34}
\end{subfigure}
\hfill
\begin{subfigure}[b]{0.475\textwidth}
\centering
\includegraphics[width=\textwidth]{Example-Image}
\caption[]%
{{\small Network 4}}
\label{fig:mean and std of net44}
\end{subfigure}
\caption[ The average and standard deviation of critical parameters ]
{\small The average and standard deviation of critical parameters: Region R4}
\label{fig:mean and std of nets}
\end{figure*}
\end{document}
Of course you would drop the \usepackage{mwe}
and substitute your pictures back in at appropriate scaling.