I'm very impressed by your ambition. When I was your age, it was a miracle if simply turned in my homework on time. I have to agree that No Limits is a very worthwile investment. The CAD-type interface is very easy to use, and the physics simulator (gravity, g-forces, etc.) is very accurate. However, they do not have 4D coasters available. However, there is a similar game called Scream Machines (http://www.wounded-heart.com/) that does have a 4D design. I've only owned No Limits, so I do not know how they compare. However, from what I've seen, the design and simulator modules seem to be pretty much the same (though the graphics seem crisper and cleaner in No Limits).
Good luck with your project.
-Doug