I would start looking at the Mathematics Extension 1 Course if you haven't, because the two courses can be interrelated depending on the topic.
The 3U Vectors course is good background knowledge walking into the 4U course, and there isn't much difference between the two (i.e you still do things like Dot Product, Geometric Proofs, etc..) except in 4U you also look into things like the vector equation of a line or a sphere. Integration is another good example of where the two courses are interrelated. The 4U Integration Course expands on the 3U Integration Course, with topics such as Integration by Parts and Recurrence Relations being assessed in the 4U course. However, in 4U you also do topics such as Integration By Substitution, etc..
Otherwise, I'd start with the Complex Numbers topic (as that is in most cases the first topic done), but take it slow if a concept doesn't stick right away. You still have your HSC year ahead of you, meaning there's an abyss of time to develop and master your understanding on particular topics. 4U was a slow start for me (coming from someone with mediocre marks for the Prelim Extension 1 course), but over the course of this year I feel like I've come to terms with the subject.