Another note that I've been meaning to mention for some time: Under the current combo system, having four different combo numbers is, mathematically speaking, no different from having just two, since both 1 and 3 can combo into either 2 or 4, and both 2 and 4 can combo into 1 and 3 (there's no effective difference between a 1 or a 3 card; similarly, there's no effective difference between a 2 or a 4 card).
You could make the combo system simpler (without changing the current probabilities) by just using 1 and 2 for the combo numbers.