you should do it if u really love maths, but only if you have the right attitude. depending on what kind of student you are depends on how easy or hard you will find 4unit. But the thing ive learnt most this year is you CANNOT lose faith in yourself. i personally find 4unit pretty demanding... but i never expected it to be a walk in the park wen i chose to do it... it is 4unit after all!!
I have noticed though that whenever i lose faith in my ability, i dont do as well, and whenever i feel confident that i can do it, i do much better.
maths requires logic as you well know, so if u choose to do 4unit dont freak out in the first few weeks. give it a chance, because there is plenty of time to drop.
i think that if u love maths as much as it sounds like you'll do brilliantly!
and the topics just sound scary (well to me anyway!) but half the time they arent that bad!!
so wen u hit complex numbers first up (most likely) dont worry coz its prolly one of the easiest topics!
good luck!!