what plus what equals 4?

(1 + 3i) + (3 - 3i)

π + (4-π) = 4

x+y=4

-2+6 = 4
-1+5 =4

1 plus 3 = 4,
2 plus 2 = 4.

0+4

There are an infinite number of answers but if you have not learned negative numbers or fractions yet the answer is
1 + 3
2 + 2
4 + 0

Every pair of numbers or expressions x and y such that x+y=4.