Here's an answer using the spath3
library, which works by splitting the elliptical and line paths where they intersect and then constructing a new path by welding together pieces of the resulting paths.
\documentclass{article}
%\url{https://tex.stackexchange.com/q/652890/86}
\usepackage{tikz}
\usetikzlibrary{intersections,spath3}
\begin{document}
\begin{tikzpicture}
\path[spath/save=circles] (0,0)
circle[x radius=4, y radius=2]
circle[x radius=3, y radius=1]
;
% the overlay key means that these don't add to the picture size,
% also, a single path here simplifies the code later
\path[spath/save=lines,overlay] (10:5) -- (0,0) -- (-10:5);
% split the paths where they overlap each other
\tikzset{
spath/split at intersections={circles}{lines},
% save the components of the paths
spath/get components of={circles}\circlesCpts,
spath/get components of={lines}\linesCpts,
}
\fill[
magenta,
% takes a little experimenting to get the order right ...
spath/use={\getComponentOf\circlesCpts{3}},
spath/use={\getComponentOf\linesCpts{2},weld},
spath/use={\getComponentOf\circlesCpts{6},reverse,weld},
spath/use={\getComponentOf\linesCpts{4},weld},
];
% draw the ellipses now so that they are over the top of the
% filled region
\draw[ultra thick, spath/use={circles}];
\end{tikzpicture}
\end{document}
