  We study the mixed Dirichlet-Neumann problem for the Laplace equation in the
complement of a bounded convex polygonal quadrilateral in the extended complex
plane. The Dirichlet\,/\,Neumann conditions at opposite pairs of sides are
$\{0,1\}$ and $\{0,0\},$ resp. The solution to this problem is a harmonic
function in the unbounded complement of the polygon known as the
\emph{potential function} of the quadrilateral. We compute the values of the
potential function including its value at infinity.
