\documentclass[a4paper]{article}

\usepackage{pgfplots}

\usepgfplotslibrary{patchplots}

\begin{document}

\begin{tikzpicture}
	\begin{axis}
	\addplot3[patch,colormap/blackwhite,patch type=biquadratic,shader=interp] coordinates {
		(0,0,1)
		(1,0,0)
		(1,1,0)
		(0,1,0)
		(0.5,0,0)
		(1,0.5,0)
		(0.5,1,0)
		(0,0.5,0)
		(0.5,0.5,0)
	};
	\addplot3[surf,opacity=0.5,shader=interp,domain=0:1]
		{4*(1-x) * (1-y) * (x-0.5)*(y-0.5)};
	\end{axis}
\end{tikzpicture}
\end{document}

