\begin{figure}[H]
  \begin{center}
  \unitlength=4pt
    \begin{picture}(20, 26)(0,-3)
  \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=3}
    \thinlines
    \node[Nmarks=if,iangle=90,fangle=90](A0)(0,20){$1$}
    \node(A1)(20,20){$2$}
    \node(A2)(20,0){$3$}
    \node(A3)(0,0){$4$}
    \drawedge(A0,A1){$a$}
    \drawedge(A1,A2){$a$}
    \drawedge(A2,A3){$a$}
    \drawedge(A3,A0){$a$}
    \drawloop[loopangle=180](A0){$b$}
    \drawloop[loopangle=0](A1){$b$}
    \drawloop[loopangle=0](A2){$b$}
    \gasset{curvedepth=-4}
    \drawedge[ELside=r](A3,A0){$b$}
    \end{picture}
  \end{center}
  \caption{Un automate déterministe.}
\end{figure}