C3 and C4 aren't that hard. They are quite straightforward, they just build on C1 and C2. It's very possible to get 100/100 in both.
With C4 its all about learning the different integration techniques and being able to apply them. Integration is practically in every question, bar one about vectors.
I actually found C4 easier than C3, although since they are both mainly integration/differentiation, one is a reverse of the other.
About learning lots of trig equations, the trick is to learn to derive them, which makes life far easier