Its +20 from 2-3, 3-4, whereas its +40 dmg from 1-2. This is still an alright build and will kill SOME mages, but I doubt it will kill the better mages as u don't have the frost combo and relying on drain which has high mana output isn't very clever.
Normally I'd save my drain till their full light debuffed, then do one full combo and stick mana regen on at 1/4 mana. Whereas u would constantly have to use it to keep them on edge, which it wouldn't really untill they're debuffed.