How do i find the finite approximation to estimate the area using the lower sum of 4 rectangles for f(x)= 4-x^2 between x=-2 and x=2?

The primary rectangle would be from - 2 to - 1. 
f(- 2) = 0 
f(- 1) = 2 
this rectangle has 0 stature, subsequently it has no territory. A = length * width = 0. 

second rectangle from - 1 to 0 
f(- 1) = 2 
f(0) = 4 
this rectangle has stature of 2 (the base). range = stature * width = 2 * 1 = 2

 third rectangle from 0 to 1 is the same 
A3 = 2 
fourth rectangle from 1 to 2 is the same as the first: 
A4 = 0 * 1 = 0 
Add up to range = A1 + A2 + A3 + A4 = 4

Idon’t know how to do that