I still stand by my requisition of finishing 2U topics first before attempting 4U.
This is excellent advice!
If you are wanting to excel in 4u, your best way of doing this effectively is to get ahead in 3U. If you can cover 2U/3U Calculus and Trig early, that is going to give you a heap of time next year to consolidate and refine that knowledge as well as importantly give you the time to bury yourself in the 4u work when you are going to make best use of your time doing it. Doing complex numbers early really isn't going to help you that much (graphing however would be the best if you wanted to do a 4u topic early because so much of the syllabus is reliant on graphical understanding and applications).
But yeah, my advice is get ahead with your 3u stuff.
Best Regards
OldMathsGuy