Band 8 are very advanced users, able to do anything- if not to the standards of accuracy of a native speaker, then close enough. They should waltz through Proficiency if they're getting that sort of grade. When I was teaching pre-sessional, 7.5 was considered enough to do a post-graduate degree in Law, though I believe there may be a bit of grade inflation going on internationally and the bar may be 8 now, but if they can cope with legal English comfortably, they can cope with pretty much anything. Science courses tended to ave lower requirements (around 6)
Band 4 are ropey intermediates and barring miracles are probably not going to make the language grade for most courses.