\documentclass[a4paper, 12pt, fleqn]{article}
\usepackage{graphicx, fancyhdr, url, abstract}
\usepackage{amsmath, amsthm, amssymb, amsfonts, geometry}
\usepackage{caption, subcaption}
\usepackage{booktabs, dcolumn, multirow, float, rotating, morefloats, here}
\usepackage[round, longnamesfirst]{natbib}

\setlength{\topmargin}{20mm}
\addtolength{\topmargin}{-1in}
\setlength{\textheight}{235mm}

\begin{document}

\def\mySUBFIGURE#1#2#3{
  \begin{subfigure}{.475\columnwidth}% .475:できるだけ Overfull \hbox を出さないため
    \centering
    \FBOX{\includegraphics[#1]{#2}}
    \caption{#3}\label{subfig:#3}
  \end{subfigure}
}

% 読み込む eps に枠をつける場合は fbox の行を後にする
\def\FBOX#1{\fbox{#1}} % 枠有り
\def\FBOX#1{#1} %%%%%%%% 枠無し

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{オリジナル}
\begin{figure}[H]
  \mySUBFIGURE{width=\columnwidth}{MM.eps}	{MM-1}
  \mySUBFIGURE{width=\columnwidth}{SD.eps}	{SD-1}\\
  \mySUBFIGURE{width=\columnwidth}{Skewness.eps}{Skewness-1}
  \mySUBFIGURE{width=\columnwidth}{Kurtosis.eps}{Kurtosis-1}
  \caption{Sample Stats --- オリジナル}\label{fig:sample-1}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{オリジナルの eps ファイルを枠付きで表示}
{
\def\FBOX#1{\fbox{#1}} % 枠有り

\begin{figure}[H]
  \mySUBFIGURE{width=\columnwidth}{MM.eps}	{MM-2}
  \mySUBFIGURE{width=\columnwidth}{SD.eps}	{SD-2}\\
  \mySUBFIGURE{width=\columnwidth}{Skewness.eps}{Skewness-2}
  \mySUBFIGURE{width=\columnwidth}{Kurtosis.eps}{Kurtosis-2}
  \caption{Sample Stats --- 枠付き}\label{fig:sample-2}
\end{figure}
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\def\FBOX#1{#1} % 枠無し...検証のため幅を小さくしてみる
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{ファイル表示幅を狭くする}
\begin{figure}[H]
  \mySUBFIGURE{width=.7\columnwidth}{MM.eps}      {MM-3}
  \mySUBFIGURE{width=.7\columnwidth}{SD.eps}      {SD-3}\\
  \mySUBFIGURE{width=10mm}          {Skewness.eps}{Skewness-3}
  \mySUBFIGURE{width=10mm}          {Kurtosis.eps}{Kurtosis-3}
  \caption{Sample Stats --- eps ファイル表示幅を狭くする}\label{fig:sample-3}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{2x4 から 2x8}
\begin{figure}[H]
  \mySUBFIGURE{width=\columnwidth}{MM.eps}      {MM-4}
  \mySUBFIGURE{width=\columnwidth}{SD.eps}      {SD-4}\\
  \mySUBFIGURE{width=\columnwidth}{Skewness.eps}{Skewness-4}
  \mySUBFIGURE{width=\columnwidth}{Kurtosis.eps}{Kurtosis-4}\\
  \mySUBFIGURE{width=\columnwidth}{MM.eps}	{MM-4-2}
  \mySUBFIGURE{width=\columnwidth}{SD.eps}	{SD-4-2}\\
  \mySUBFIGURE{width=\columnwidth}{Skewness.eps}{Skewness-4-2}
  \mySUBFIGURE{width=\columnwidth}{Kurtosis.eps}{Kurtosis-4-2}\\
  \caption{Sample Stats --- 2x4 から 2x8}\label{fig:sample-4}
\end{figure}

\section{順番を入れ替えてみる}
\begin{figure}[H]
  \mySUBFIGURE{width=\columnwidth}{Skewness.eps}{Skewness-5}
  \mySUBFIGURE{width=\columnwidth}{Kurtosis.eps}{Kurtosis-5}\\
  \mySUBFIGURE{width=\columnwidth}{MM.eps}	{MM-5}
  \mySUBFIGURE{width=\columnwidth}{SD.eps}	{SD-5}
  \caption{Sample Stats --- 順番を入れ替えてみる}\label{fig:sample-5}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\def\EACH#1{\section{一つづつ #1}\fbox{\includegraphics{#1.eps}}}
\EACH{Kurtosis}
\EACH{MM}
\EACH{SD}
\EACH{Skewness}

\end{document}

