4u IS fun. It's also very rewarding. If you like a mental challenge, definitely take it. That said, it's not a total mind warp or anything. A lot of it is simply an extension of 2u and 3u stuff, so there's still a fair amount of simple things like integration, arithmetic and algebraic manipulation like in trig proofs or identities and stuff (under the guise of complex numbers), graphing, more polynomials.
I think in hindsight the biggest to advantage to having taken 4u is that 4u is an awesome background for maths at uni, even if you don't do advanced/honours maths in first year. Take it and you'll thank yourself next year.