Which level 4 monster is the best?

There's no point in arguing. Vampires help Necropolis in most situations; they have good speed and damage! And most importantly, they drain life.

Vampire lords are, of course, the strongest, but let's be honest: their damage is actually one of the lowest (only energy elementals have less), their speed is normal, but there are faster units, like pegasi. Vampire lords are strong, firstly, because of their ability to drain life, secondly, because of their retaliatory strike, thirdly, because of their ability to fly. And finally, and in combination with everything else, their speed.