How much/how often is he eating during the day? Sometimes if they're going for longer stretches during the day, they make up for it at night.
At 4 months, my son was only waking once a night, and for us, things started getting worse at 5 months. That's when he started waking multiple times a night. I think it was developmental though, because slowly but surely, over weeks he went back to his once a night wake. Of course, that was until he started teething at 10 months...