it really depends. If you are really good in 3U, then you shouldnt really be studying, as everything is past knoledge anyway. What you should be doing for 3U however, is doing lots and lots of questions, same for 4U once you learn the stuff. I think that 4U should still be given a considerable amount of time, since it is a good scaler, and it is 4U i guess (or is it 2?). But then again, ifyou are doing 10units, then you should have even spread for everything. I do twelve units, so i forego my physics time for maths, and concentrate on 4U maths papers, rather then 3U papers, as they tend to be very repetitive and pretty easy.
BUt all in all, as you get closer to the test, all you should be doing is tests and questions, i find that many 4U questions are different, so it is good to practice.
doing questions are the main thing to going good i think, lots of questions.