\documentclass{article}

\usepackage[all]{xy}

% Some obscure definition to create a circled node within xy.
% The definition is made by Freek Wiedijk who prefers to do his definitions
% in TeX instead of LaTeX, which explains the \def instead of \newcommand.
\def\node{*++[o][F-]}
\def\fnode{*++[o][F=]}

\begin{document}

% Add the missing transitions.
\[
\xymatrix{
\ar[r]
& \node{q_0} \ar[drr]_<<<<<{b}
&
& \node{q_1} \ar[dll]^<<<<<{b}
\\
& \fnode{q_3} \ar@/_0.5cm/[rr]^b
&
& \node{q_2} \ar@(ur,dr)^b
}
\]


\end{document}
