If there are spots available, you can still ask to go in and I'm pretty sure that if they have space, they must let you in assuming MX1 is atleast done.
There are some schools like Malek Fahd that may have certain criteria you must meet to do 4u. First you's need to be in the top accelerated class so they let you sit the 2u HSC then you have to score a Band 6 in the 2u HSC (if you dont then you cant do 4u period) and do well in your 3u prelim exams (if you dont they strongly advise you to not do 4u). So depends on your school. But honestly they shouldnt force you not to do 4u.
Read this thread I've just posted: http://community.boredofstudies.org...tment-not-being-able-pick-up-3u-4u-maths.html If your school seriously doesn't let you do it( even after you've begged, cried, threatened etc) and you still want to do it, you can do it outside. Doing it outside, you won't be under your school's name so they can't do anything about it.