Which level 4 monster is the best?

Not just against Crusaders (although that's my favorite unit in the game, but we need to be realistic). Ogres can also put up a decent fight against vampires – they have a lot of hit points, high attack power, plus Bloodlust, while the rest are just easy prey.

Ogres are cool guys, I agree. Their damage is almost twice as high as that of vampires. But they will still lose to vampires, and Crusaders won't be able to put up much of a fight against them. Personally, for me (I checked this by fighting between level 4 units), 1st place goes to the Vampire Lord, 2nd place to Crusaders and Archmages, and 3rd place to the Ogre Mage (I'm not taking neutral units into account). That's how it is...:)