well, i'm on the ccea board, and personally i found both easy enough. I feel that c4 is slightly easier than c3, but the exam was harder. Still, i feel i did much better in the c4 exam than the c3.
It was explained to me like this, c3 is easier for the people who revise a lot, even if they aren't as good at maths, whereas c4 is easier for the people with a natural ability in maths, even if they dont revise as much.