here is an easier way to think about it.
lets say there is 100 psi pressure in the brake line. if we double the
size of the piston, we double the pressure on the brake pads.
if we have 2 piston calipers, and convert to 4 piston, but the total
area is the same, we have the same pressure squeezing.

HOWEVER, with 4 pistons, we might have more consistent, and even
pressure. Which means we could get a higher clamping force before the
brakes lock up.