Which level 4 monster is the best?

Vampires defeat some units of level 6, that's a fact. One-on-one, vampires will undoubtedly lose, but in a game, there are never situations where there are as many vampires as there are level 6 units; there are always fewer level 6 units, plus many die in battles, while vampires rarely lose any due to their life-draining ability. Therefore, vampires will not lose to wyverns, cyclops, or manticores. However, cavalry, death knights, psychic elementals, and, God forbid, nagas will shred the vampires, especially nagas.