What is the best way to go about getting into Morthal to talk to Falion about curing Stage 4 Vampirism. I've been able to sneak in his house during the day but he becomes hostile. How and when can I find him in the city without having to kill all the citizens?

You're going to have to feed to reduce your Stage 4 vampirism. Once you're at a lower stage, Falion will not be hostile.

Find a sleeping NPC and sneak up on him or her - the option to feed should appear. If you have a follower, you can order him or her to sleep in a nearby bed and feed that way.