What do you do when your 4-month-old baby starts to bite your nipples?

try not to react from it and stop breastfeeding for awhile then look to him/her showing that it's not good, then continue feeding, if it happens again, repeat the same process,I know it's hard not to react because it hurts big time and sometimes our lo's are smiling and looking what will be our reaction and making fun of it. I hope this helps