Effectively, in 4e, circles equal squares. Which means that pi = (complicated equation equating r squared with a formula to determine the [variable] distance from the center of a square to it's perimeter). Think about what that does to geometry, let alone physics.
It's not complicated at all: A 5-foot-radius "circle" in 4e is a 2x2 grid of 5 foot by 5 foot squares, with a circumference of 40 and an area of 100, so from 2*pi*5 = 40 and pi*5*5 = 100 we know that pi = 4.
Thats part of the problem. You see, your answer is correct (giving pi for the shortest radii of the square, but while one radius is 5, the diagonal radius is 7.07 (the square root of 50)(See Pythagorean theorem). Or, if we plug it back into the area calculation, tells us that 100=pi*50, or pi=2.
Thus, pi is a range of 2 through 4, with an interesting wave function based on where the degree of the radius is in relation to a designated 0 angle on the square (for simplicity I put a 'short radius' (or five in our case) at the 0 point).
And because its a wave function, the best way to express pi was with a trigometric function, which is where it gets complicated. This is the portion that I've forgotten at some point in the past. It's been a while since I've used more complicated math than arithmetic or plug-and--play algebra.