from the amount of 4u I've done (75% of the course ish), 4unit is really about critically thinking about questions that examiners can really just pull out of their ass. there are obscure questions that really fuck students over, but while I'm not pretending to be a state rank student or anything, I find continually spending your time on revisiting concepts doing past HSC and trial papers and even doing textbook q's really really help the understanding of any math concept, in both 2u and 3u.
with that being said, it is completely and entirely up to you, and only you to force yourself to do such practise.
I find 4u really differs from 3u as 4u you have to be able to forecast the answers or the steps to the answer and know when to stop, whereas 3u you can keep bashing at the question and it'll come out sooner or later. but remember 3u is a 2 year course which means in your HSC they can assess you on prelim shit. that's why 3u IHMO is that much easier than 4u.
in all do 4u if you really think you have the dedication, if you can truly appreciate mathematics, not just do it "for the high scaling" and if you are able to forecast answers in 3u maths. I'm not boasting but when I do tests I rarely cross out entire questions, but I see see of other students cross out entire questions. so yeah.
complex - new concept of numbers
curve sketching - extended remix of 3u curve sketching
polynomial - harder 3u poly
conics - extended remix of parametrics
integration - harder 3u integration with more techniques
mechanics - motion bullshit
volumes - self explanatory... integration and shit
harder 3u - the hardest questions on circle geo, binomial, and etc.
if you think you're up for those topics above then give 4u a shot