Ok so i know that you can join the College of Winterhold at stage 4, if you charm Faralda, and its members will not attack you either (though i heard if you feed and later go back to stage 4, they will turn on you).

So does this apply to the Companions group? If using the the charm spell/vampire seduction...could i get into the Companions even on stage 4?

Also i know that if you complete someones quest....they won't attack you, even if your at stage 4. So since i completed a quest (the one early on in the game which leads to the first dragon attack) for the Jarl of Whiterun, and also another quest for Farengar (gave him that dragonstone), that means they won't attack me right? And if they don't attack me, i was wondering if this would extend to the guards in dragonsreach as well.

does anyone have an answer?

well, you have to become a werewolf to be in the companions, and i dont think you can be a werewolf and a vampire at the same time. so im gonna say no. in fact, becoming a werewolf is a good way of curing vampirism.

im not sure about this quest youre referring to, since ive never tried vampirism.

I think at stage 4 of Vampirism, everyone will attack you on sight regardless of your deeds for certain NPCs. The only way is to use the seduction power or some other calm spell.