I've reached stage four vampirism and everybody is attacking me.

Trouble is, I'm supposed to go to an innkeeper, who will tell about some guy called Fallion. Yet he attacks me too. I can't feed off NPCs, either.

Somebody help me, please!

Once you hit stage 4 vampirism without Dawnguard installed, everyone will be hostile to you. You'll need to feed off of someone before you can get cure quest. Camila or Lucan in Riverwood would probably work since there aren't many people around and their shop is pretty close to the edge of town.

Edit: Actually, Hunter's Rest south of Rorikstead would be easier. There are only two people there and I you don't manage to sneak up on them, just use a pacify spell.

Cool, thanks for that.