Ok so there came a time when one of my characters who was a vampire progressed to stage-4. He ended up in the riverwood trader, and to my suprise i didn't have to use charm on Camilla and Lucan even though i was at stage-4, they just didn't attack me. I came to the conclusion that if you complete a quest for someone they won't attack you on stage-4. Alvor didn't attack me either, since i did complete a quest for him as well. And this was not the only time in which i came across people who i had completed quests for, who didn't attack me regardless of my stage of vampirism.

However today, i used the same character who i fast travelled to riverwood, and when he became stage 4 and went into the riverwood trader, the people there attacked me. What gives? They didn't attack me last time i was stage 4 at that place. Is it a bug?

Perhaps it was something that was changed in version 1.6.