begin{figure}[h]
  centering%
  begin{subfigure}{0.5textwidth}
    centering
    includegraphics[width=linewidth]{1.eps}
    caption{$v_l^* - l$}
  end{subfigure}%
%  hspace{4em}%
  begin{subfigure}{0.5textwidth}
    centering
    includegraphics[width=linewidth]{2.eps}
    caption{$sigma_l^* - l$}
  end{subfigure}

  begin{subfigure}{0.5textwidth}
    centering
    includegraphics[width=linewidth]{3.eps}
    caption{$v_b - l$}
  end{subfigure}%
%  hspace{4em}%
  begin{subfigure}{0.5textwidth}
    centering
    includegraphics[width=linewidth]{4.eps}
    caption{$sigma_b - l$}
  end{subfigure}

  caption{}
  label{fig:ov}
end{figure}