If you become a vampire and reach stage 4, without buying a house or having a follower, it can be very frustrating trying to revert to stage 1.
Just in case you don't know, in order to go back to stage 1 you have to drink someone's blood while they're asleep. However, during the final stage, everyone will attack you on sight.
There is a place where a sleeping person sometimes is and you don't have to lockpick! It does require a lot of stealth to get to, though.
South of Solitude there is some sort of shipping building. Inside, there is a house near the roof of the building/cavern.
Use your sneaking ability, with a little skill and luck you should be able to feed! I've done this before and it worked.
Hope this helps! :D