There are 36 different possible outcomes for rolling the number cubes. To get a sum of 4 you would need to roll a 1 and 3 (2 possibilities, each cube could roll a 1 or a 3) or a 2 and a 2 (1 possibility, each has to roll the same number). Since there is no 0 or negative numbers, these are your only possibilities. 3 out of 36 possibilities is also equivalent to a 1/12 chance.
