I attempted to answer the following question as follows:-
I computed the potential energy of four electrons located at the apices of a tetrahedron at a distance of 2.0 Å = 6.92e-18 Joules.
I computed the potential energy of four electrons located at the four corners of a square with edges of 2.0 Å =4.61e-18 Joules.
As four electrons located at the four corners of a square have lower potential energy than the potential energy of a tetrahedron, it is more stable.