Problem
The sum of the interior angles of a quadrilateral is equal to four right angles.
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Solution
As shown in the diagram above, if you draw a diagonal $AC$ on a quadrilateral $ABCD$, it can be divided into triangles $ABC$ and $ACD$.
Since the sum of the interior angles of a triangle is equal to two right angles, the sum of the interior angles of the two triangles is four right angles.
This also applies to the case of a concave rectangle, as shown in the following figure.
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Reference Teiichiro Sasabe (1976) The Encyclopedia of Geometry (2nd edition), Seikyo-Shinsha, p.41.