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?

He's definitely not getting enough sleep if he is being fed so frequently. At 4 months old you can definitely stretched his feeding to 3.5-4 hourly. The best way to stretch his feeding frequency is do it at the start of the day - assuming at night he doesn't wake up so frequently and do the EASY cycle Eat, Activity, Sleep and Your Time. Make sure each feed is a full feed. If he's just snacking then you won't be able to stretch his feeding. All the best and sleep well!