\begin{tikzpicture}[>=latex,join=bevel,]
%
\node (a) at (33bp,106bp) [draw,ellipse] {$a$};
  \node (b) at (5bp,57bp) [draw,ellipse] {$b$};
  \node (c) at (33bp,57bp) [draw,ellipse] {$c$};
  \node (d) at (35bp,7bp) [draw,ellipse] {$d$};
  \draw [->] (a) ..controls (26bp,94bp) and (19bp,82bp)  .. (b);
  \draw [->] (a) ..controls (33bp,94bp) and (33bp,82bp)  .. (c);
  \draw [->] (b) ..controls (12bp,44bp) and (20bp,32bp)  .. (d);
  \draw [->] (d) ..controls (42bp,24bp) and (50bp,45bp)  .. (47bp,64bp) .. controls (45bp,73bp) and (41bp,83bp)  .. (a);
%
\end{tikzpicture}