Although it would be wonderful, there just isn't a clear boundary between what you 'need to know' for the MCAT and more advanced material, especially in the biological sciences. The transition from level 2 to level 3 in the question server at WikiPremed represents my own subjective sense where there is a shift from black to grey, above which it's still good to know the term but that it's okay not to stress, to have a learning goal of 'familiarity' or 'exposure' instead of absolute fluency.
Some students go into the MCAT knowing that there are neurons and 'glial cells' in the central nervous system while others know about ependymal cells, astrocytes, and oligodendroglia. While you 'need to know' the former, knowing the latter will make you more surefooted with this content, and the second group is 'advantaged'. Knowing more correlates to doing better. You are more comfortable in the passages, so there isn't some misty netherworld over the horizon containing everything you've never seen, so I try to present the material with some nuance. Advanced terms the MCAT may throw at you in passages is a bigger set than those which could govern a fact based questions and as you move into very advanced regions of knowledge the imperative to know the term versus just having been exposed to it grows less, and so you don't need to stress so much about level 4 terms at all, but reading through them will help you anyway.
Basically, with the WikiPremed question server, as you move from level 1, 2, 3 to 4, just gradually let yourself off the hook as you go higher and higher. With the level 4 terms, it's good to get a bit of exposure. To try the term out. Give yourself a sense of what expert knowledge feels like, but don't stress about it. When I was making the glossary, I put the transition from level 2 to level 3 where the change went from 'need to know' to 'should know'.