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?

His sleep may be intermittent as he wakes every 1.5 hours to feed? Do you mean he wakes every 1.5hr throughout the day even at night? For a 4mo I think you can try to stretch his frequency to every 2.5-3 hours. In this way he gets a longer sleep and the quality of sleep overall will be better. How many hours of sleep does he have in total every day. At 4mo he should still be having 13-16 hours of sleep per day and wakes up lesser to drink at night altho every baby is still different.