\documentclass{article}
\usepackage{graphicx}

\begin{document}

    \begin{center}
    \begin{tabular}{cc}
    \includegraphics[width=.2\vsize]{chou3.pdf} & 
    \includegraphics[width=.2\vsize]{takao2.pdf} 
     \\
    \includegraphics[width=.2\vsize]{yamashita2.pdf} & 
    \includegraphics[width=.2\vsize]{hane2.pdf} 
    \end{tabular}
    \end{center}

\end{document}

