Two ways to solve this:
1) Notice that this is a 45-45-90 triangle, meaning the ratio of sides and hypotenuse is 1:1:sqrt(2)
Since the legs are 4, that would mean the hypotenuse would be 4*sqrt(2)
2) Pythagorean theorem: a^2 + b^2 = c^2
4^2 + 4^2 = 32
Sqrt(32) = 4*sqrt(2)
