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That would turn it into an octagon.
If you mean the base has 4 sides then the pyramid is a square based pyramid and has 5 corners. If you mean the pyramid has 4 faces then the pyramid is a tetrahedron with a triangular base and has 4 corners.
An example of a 90 degree angle is any of the four corners of an A4 page.
By cutting off two of its corners which will leave you with 2 triangles and 1 pentagon.
To do something in way that is the cheapest and easiest.