S.B.
You poor thing. Trust me, I know how much this hurts! Their little teeth are like razors.
The first time my son bit me, I just about went throught the ceiling.
It was pretty bad. My mom told me that when a baby starts to bite, it's time to be done with nursing, but I didn't feel ready to give it up just yet.
Give yourself a couple of days of healing time and just nurse on the other side. That's what I did. I kept my sore breast exposed to the air as much as possible so it could heal more quickly.
The trick is, even this young, is if the baby bites or begins to, unlatch and lay the baby down for a few minutes. He may cry, but that's okay. Then, give it another try. If it happens again, take the breast away and give him a teething ring to chomp on for a few minutes and offer the breast again.
Believe it or not, they really do get it figured out that mommy isn't a chew toy. Often, my son was perfectly happy with a teething ring because he wasn't really interested in nursing right that minute anyway. He wanted something to bite down on and it wasn't going to be me!
At 4 months saying NO or DON'T BITE won't really register, but taking the breast away and offering something to bite on will. If they're truly hungry, they realize they won't get any milk out of the teething toy, and believe me, fussing a bit won't hurt your baby.
I went from knowing I might have to stop nursing if one of my nipples had to get stitched back on to happily nursing my son for 15 months.
I wish you the best and hope you're feeling better soon!