All the characters have a fourth level limit (except Kitten Sith, he don't need no stinkin' level three and up Limits!) which is learned by using a certain 'Limit Break Manual' on them as an item once they've learned all their level one, two, and three limits.

However, alot of these final Limits aren't really all the characters' ultimate Limits. For example, Yuffie's Doom of the Living beats her level 4 Limit, All Creation, anyday. Barret's Angermax is much better than Catastrophe for all-out damage. And so on. But Omnislash and Highwind are still the best choices for Cloud and Cid.

Anyway, just use a guide *cough* to find these Limits.