Answer:
Assume the pair of dice are fair; if not, all bets are off!
Color one die green (G) and the other one red (R). There are 36 possibilities in throwing one green and one red die, eg (G1, R1), (G1, R2), etc; (G2, R1), (G2, R2), etc; etc; (G6, R1), (G6, R2), etc.
In the set of possible numbers from throwing a die, there are only 3 possibilities that sum to to 4: (G1, R3), (G2, R2), (G3, R1).
So the answer is 3/36, or one-twelfth.