im doing 4u at the moment, and i would definitely recommend getting a tutor... there is so much material to cover in the 4u course, but more importantly there are heaps of little tips and tricks, and if you manage to get a good tutor (as i have just gotten) you can pick up all of those tricks that your class teacher may not know...
im sure there are plenty of people who will disagree with getting a tutor, but i think it would be the best move... i go to a good private school and all of the maths teachers are pretty top teachers, but still all of the top boys (like most of the top 20 or so) still get tutored...