My 4 mths old baby keep yawning whenever he woke up from nap and his night sleep. Can I know why? He is breastfeed so he woke up very 1.5 hr. He does not get enough sleep?

It is normal for a young baby to wake up every 1.5 hours at night for feed. Every baby has a different sleep and feed schedule. If this is something that has changed only recently, it could be because he's going through a growth spurt. I would follow his cue and try to put him back to sleep if he keeps yawning. Does he have awake time that he's not yawning? period of time where he's alert? If he's constantly yawning, I'd definitely bring him to a pd or at least highlight it in the next checkup.