This is not correct. You are counting things as though order matters for the total number of ways to pick four candies from the 24 in the box. But when you are counting the number of ways for each of the cases (e.g. RRYB), you have counted the ordering as though the reds must come first, then the yellow, then the blue. You didn't take into account cases where yellow came first, then blue, then the reds, etc. (which are still cases where we pick two reds, one yellow, and one blue, etc.). So you have undercounted the desirable outcomes, so your answer is less than the true answer.