Confusing in [for loop] topic

[Sherine]

Hi,

I read a book with this below question and answer.
However I cant get the meaning of the explanation.

x = 0
n = 5
for i in range(1, n):
    for j in range(1, n):
        if((i+j) == 2):
            x = x + 2
        if((i+j) == 3):
            x = x + 3
        if((i+j) == 4):
            x = x + 4
print(x)

Output:
20

The explanation was:
First loop are 1,1 ; 1,2 ; 1,3.
Second loop are 2,1 ; 2,2.
Third loop are 3,1.
Fourth loop is nothing.
>>> So x = 2 + 3 + 4 + 3 + 4 + 4 = 20

Why there is nothing in the fourth loop?
How is the logic of this loop ?