\documentclass[a4paper]{article}

\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
% this is a matlab format with TWO segments of the same contour level.
	\begin{axis}[colormap/jet]
	\addplot[contour prepared={labels=false},contour prepared format=matlab] table
	{
	0 4
	0 0
	1 0
	0.5 0.5
	0 0
	0 4
	0 2
	1 2
	0.5 2.5
	0 2
	};
	\end{axis}
\end{tikzpicture}

\end{document}

