I wish to shade multiples regions (labelled A) that are formed by two quarter circles, making a sort of matrix.
\documentclass[10pt]{standalone}
\usepackage{pgf,tikz}
\pagestyle{empty}
\begin{document}
\begin{tikzpicture}
\draw (8.,12.) -- (4.,12.) -- (4.,8.) -- (8.,8.) -- cycle;
\draw (8.,12.) -- (8.,8.) -- (12.,8.) -- (12.,12.) -- cycle;
\draw (12.,12.) -- (12.,8.) -- (16.,8.) -- (16.,12.) -- cycle;
\draw (16.,12.) -- (16.,8.) -- (20.,8.) -- (20.,12.) -- cycle;
\draw (8.,8.) -- (4.,8.) -- (4.,4.) -- (8.,4.) -- cycle;
\draw (12.,8.) -- (8.,8.) -- (8.,4.) -- (12.,4.) -- cycle;
\draw (16.,8.) -- (12.,8.) -- (12.,4.) -- (16.,4.) -- cycle;
\draw (20.,8.) -- (16.,8.) -- (16.,4.) -- (20.,4.) -- cycle;
\draw [shift={(4.,12.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=-1.5707963267948966:0.,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(8.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=1.5707963267948966:3.141592653589793,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(8.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=0.:1.5707963267948966,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(12.,12.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=3.141592653589793:4.71238898038469,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(4.,4.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=0.:1.5707963267948966,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(8.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=3.141592653589793:4.71238898038469,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(12.,4.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=1.5707963267948966:3.141592653589793,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(8.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=-1.5707963267948966:0.,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(12.,12.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=-1.5707963267948966:0.,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(16.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=1.5707963267948966:3.141592653589793,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(20.,12.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=3.141592653589793:4.71238898038469,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(16.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=0.:1.5707963267948966,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(20.,4.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=1.5707963267948966:3.141592653589793,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(16.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=-1.5707963267948966:0.,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(12.,4.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=0.:1.5707963267948966,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\draw [shift={(16.,8.)},color=black,fill=white,fill opacity=0.1] (0,0) -- plot[domain=3.141592653589793:4.71238898038469,variable=\t]({1.*4.*cos(\t r)+0.*4.*sin(\t r)},{0.*4.*cos(\t r)+1.*4.*sin(\t r)}) -- cycle ;
\node at (6, 10) {$A$};
\node at (10, 10) {$A$};
\node at (14, 10) {$A$};
\node at (18, 10) {$A$};
\node at (6, 6) {$A$};
\node at (10, 6) {$A$};
\node at (14, 6) {$A$};
\node at (18, 6) {$A$};
\end{tikzpicture}
\end{document}

