First let us determine the total number of combinations that can be made from these pair. The number of combinations can simply be calculated by multiplication, so:
Combination = 4 * 4 = 16
Since we know that for each shirt color, there is a corresponding sock with the same color. So there are 4 matching pairs. Therefore the unmatching pairs are 12. Calculating the probability to get unmatching pair would be:
Probability of unmatching pair = 12/16 = 0.75
Interpretation: There is a 75% probability that his socks and shirt wont match.