\documentclass{standalone}
\usepackage{pgf}

\begin{document}
\begin{pgfpicture}
  \pgfpathrectangle{\pgfpoint{0cm}{0cm}}{\pgfpoint{4cm}{3cm}}
  \pgfusepath{stroke}

  \pgfpathcircle{\pgfpoint{2cm}{1.5cm}}{1cm}
  \pgfsetfillcolor{gray}
  \pgfusepath{fill,stroke}

  \pgfpathmoveto{\pgfpoint{0cm}{0cm}}
  \pgfpathlineto{\pgfpoint{4cm}{3cm}}
  \pgfsetlinewidth{2pt}
  \pgfusepath{stroke}

  \pgfpathmoveto{\pgfpoint{4cm}{0cm}}
  \pgfpathlineto{\pgfpoint{0cm}{3cm}}
  \pgfusepath{stroke}
\end{pgfpicture}
\end{document}
