How do I cure my vampirism if I'm already at stage 4 and can't talk to NPCs?
Feed. Find a sleeping human, and interact with them to feed on their blood.
This will cause your vampirism to revert to Stage I so that you can seek out a cure.
Hide in his room (down the side of the wardrobe in sneak mode) then wait for him to go to sleep drink his blood then wake him and he will cure you