Biting at 4 weeks is a new one!
I'm guessing that this is a less-than-optimal attachment issue that you are now noticing much more because your bub is getting stronger.

Some babies grab a nipple and shake to get another "let down" or milk flow started. This is normal - the trick is to stop it hurting you!

Try and get more nipple into her mouth - be especially careful about the underside, where you can't see. If you can get someone to have a look. Also make sure that she is held in close and that there is no way that her weight can bear down on your boob. Are your nipples still "normal" shaped after a feed, or do they look more like houses or a lipstick shape?