Here you go, Piston area and force.

Just for clarification, going from a 4" piston to 5" piston is a 36% change not more than 50% as stated.
4" piston equals 12.56" in area 5" piston equals 19.63" in area, 12.56 divided by 19.63 equals .64 (minus the .64 from .100 to get the .36 (36%)