The sum of the central angles of the pizza slices will be 360°. If N pizza slices are cut, each with a central angle of 90°, you have
.. N×90° = 360° . . . . . . the relationship between slices and the whole
Solve this equation for N to find the number of slices.
.. N = 360°/90° . . . . . . divide by the coefficient of N
.. N = 4