With both my kids I thought for sure they were teething around 4 months old, chewing on their fists and everything in sight, bright red rosy cheeks, drooling excessively, etc. My daughter got her first tooth at 11 months, and my son got his at 8 months, so neither of them were actually teething at 4 months old. Yours might be, it isn't unheard of to get a tooth that young, but at that age it's really hard to know what is teething behaviour and what is just developmental.