A function is defined by $f(0) = 0$ and $f(n) = f(n - 1) + 2n$ for $n > 0$. Find $f(4)$.

f(0) = 0

f(1) = f(0) + 2*1  =  0 + 2 = 2

f(2) = f(1) + 2*2  =  2 + 4 = 6

f(3) = f(2) + 2*3  =  6 + 6 = 12

f(4) = f(3) + 2*4  =  12 + 8 = 20