\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(30, 14)(-15,-7)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node(A1)(-15,7){$1$}
    \node(A2)(15,7){$3$}
    \node(A0)(0,0){$4$}
    \node(A3)(15,-7){$3$}
    \node(A4)(-15,-7){$1$}
    \drawedge(A1,A0){$b$}
    \drawedge[ELside=r](A2,A0){$a$}
    \drawedge(A0,A4){$b$}
    \drawedge[ELside=r](A0,A3){$a$}
    \end{picture}
  \end{center}
  \caption{Transitions passant par l'état $4$.}
\end{figure}