\begin{figure}[!ht]
\centering
\subfigure[{AGVs.}]{\label{mylabel1}
\includegraphics*[width=.25\textwidth]{pic3}
\hspace{3cm}
\subfigure[{Straddle carriers (SCs).}]{\label{mylabel1}
\includegraphics*[width=.25\textwidth]{pic4}
\subfigure[{Straddle carriers (SCs).}]{\label{mylabel1}
\includegraphics*[width=.25\textwidth]{pic5}
\hspace{3cm}
\subfigure[{Straddle carriers (SCs).}]{\label{mylabel1}
\includegraphics*[width=.25\textwidth]{pic6}
\label{mylabel}
\end{figure}