With reference to choosing 4u over 2u. I think from experience that anyone capable of doing well in 4u, and this is looking at about 70+/120, (not great) but they should be able to get at least 116/120 in 2u incorporating silly mistakes. Keep in mind that people having done 4u tend to get this mark first time through a 2u paper in less than half the time. I think if you're strong in a lot of other subjects and feel that you might not reach a great score in 4u, there's no real harm in dropping to 3u so you can sit the 2u paper, which you should have enough basic ability to get 116/120, which should very comfortably be scaled to above 90. There is a MYTH that when you do 4u, you automatically get 100 for 2u. Completely false, and hopefully you all know that. I think it's twisted from if you work at 4u, you should be able to get 100% if you ever sat the 2u exam.
But if 4u will be one of your strengths, it might be worthwhile keeping it, seeing its contribution will be higher than 2u for a high aligned mark. Another advantage of 4u is it builds your 3u as long as you keep at both. So you do better in the 3u exam and as a plus, it counts towards 2 units of work rather than 1 if you just do 3u.
Again if you're really willing to do all that initial work to build up your maths ability in 2 and 3u, you could do 4u and realising you'd go better if you drop and given you've got some units spare, drop to 3u and still blitz those exams fairly well. But doing this will take time away from other subjects, which you could recover, but I'm not making any promises.
