Well there is definitely a big step up from 3U to 4U, 4U is one of those subjects where if you have seen a question before or anything similar you are at an advantage because a lot of people won't be able to do it. 4U is really technique based, more than logical thinking imo. For example Inequalities in 4U a lot of them just have ways that make sense in the end, however you may not think of that method during the heat of an exam.Vectors is generally really easy, where they can throw curveballs is when they ask you 3D Equation of A Curve and Spirals as most people neglect that so understand how that works. Integration is easy, however they can't just assess you on calculating the integral for every single question as everyone would get that right, so they step it up to reduction formula, proving the formula is easy but they could integrate some of the previous topics like Proofs to make it harder. Complex this topic can be really easy, and really hard, make sure to go over all the various types of questions for Complex in Trial papers as there are many curveballs. Proofs is really hard as well. The main way to get good at 4U, I wouldn't say is about being really smart its just about the exposure to every single type of question there is by doing THSC past papers.