These threads are annoying.
You haven't even finished the 2u HSC course, your school is most likely going to have 1 teacher teaching you maths. That maths teacher will not neglect 2u, and most of maths will primarily be 2u and 3u in the earlier parts of year 12. There may be days where your teacher might decide to do 2u instead of 4u, so you WILL finish the course.
Also it is 4u for a reason, it contains the 2 unit course, extension 1 course and extension 2 course. 2 + 1 + 1 = 4.
With 4u when you get harder 3u questions, most will contain 2u stuff in it as well.