I'd say zoro's power level is weird right now. Technically speaking both luffy and zoro have more power than they can actually manage. Luffy's G4 drains his haki while his G5 drains his stamina. Both incapacitate him after a while (though G5 seems more permissive). Zoro in turn has learned to overuse his haki with his swords and seems to get drained pretty quickly (though we have not seem him drained yet). Both have a limited time frame over which they can go at it at full strength. Zoro's is shorter but I would still argue at that point he can more or less hold his own against a yonko. Of course, zoro does have some major gaps in his skill with haki. He can't use internal destruction, he can't use future sight, he has no control over his cladding. As far as I can tell kaido could use at least haki emission, future sight and was excellent at cladding. Big mom shows similar stuff but it's unclear if her perception was at the level of future sight. Shanks is presumably capable of all known techniques plus his technique to shut down enemy's observation haki.

Quite funny considering how much that word is used today but in a whole other meaning.