Q:

Add \hrulefill in the toc at a specified position

For a more visible separation between starred chapters, I would like to have a horizontal line (e.g., \hrulefill, or alternative) appearing in the table of contents at determined positions, in the MWE betweem the third and six chapter, maybe using \addtocontents{toc}{}{}. How could I do this ?
\documentclass{report}

\begin{document}
\tableofcontents

\chapter{Chapter 1}
\chapter{Chapter 2}

% Insert \hrulefill in the toc here

\chapter{Chapter 3}
\chapter{Chapter 4}

\end{document}

Example here:

A:

\documentclass{report}

\begin{document}
\tableofcontents

\chapter{Chapter 1}
\chapter{Chapter 2}

% Insert \hrulefill in the toc here
\addtocontents{toc}{\protect\hrulefill\protect\leavevmode\par}

\chapter{Chapter 3}
\chapter{Chapter 4}

\end{document}

Or better:
\documentclass{report}

\begin{document}
\tableofcontents

\chapter{Chapter 1}
\chapter{Chapter 2}

\addtocontents{toc}{\protect\contentsline{chapter}{\protect\rule{\linewidth}{2pt}}{}}

\chapter{Chapter 3}
\chapter{Chapter 4}

\end{document}

