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?

You might want to read up sleep training books like Babywise, Save our Sleep etc. Sleep training is not just as simple as telling you in a few sentences what to do. It involves educating yourself about baby and sleep. If you don't have time and still feel clueless after reading the books, you may want to engage the help of baby sleep experts like myself to get your baby to sleep well and through the night again. Think of baby sleep expert like a personal trainer, you may know what to do to keep your body fit and strong, but you need someone to come up with a plan and make sure you stick with it to achieve your goals. All the best and sleep well!