Dozens of ads for sleep consultants and many parenting magazines warn parents about the "four month sleep regression." But does it really exist? Best you ask your doctor but here is my observational opinion on this topic. I propose it is a made-up term used to explain why a baby who has not yet learned self-calming skills and who is therefore being helped to go go sleep - and who is still receiving night feedings at the age of 4 months... begins to awaken more frequently overnight and takes longer and longer to put to sleep at bedtime and for naps.
Many families report that to me: "He was a pretty good sleeper at 2- 3 months, needing only a quick breastfeed to put him to sleep and only 2 feedings overnight. Then he hit 4 months and he is waking now 4-6 times overnight wanting a feed and it's taking longer, bouncing, etc. to get him to sleep for naps and nights.!" Parents begin to wonder why this consistently happens around the 4 month mark.
I believe It happens because parents haven't known how to begin to shape their baby's sleep before by the 2-3 month mark. They have not yet helped their baby learn how to fall asleep on her own, nor have they known to stop night feedings overnight (11 hours) as soon as the 12 lb. mark was reached. Help to fall asleep and feeding overnight become huge expectations by 4 months of age. These habits are now very strong and harder to break.
Developmentally at 4 months babies experience a burst of social interest. So if a 4 month old is still getting even one overnight feeding, this new social development encourages him to wake up more often for the reward of seeing a parent and getting fed. Parents wrongly think this means a growth spurt is happening so they up the feedings. (This is not necessary because a baby can meet any growth spurt just by feeding a little longer during the normal daytime feeds.) Very quickly baby begins to look forward to the overnight visits and leans to "call out" more often for the thrill of these social occasions.
It takes longer to help your baby fall asleep for the same reason. So you rock and rock and bounce and then feed more and very quickly everyone is completely exhausted.
My advice is to do Sleep Shaping by 2-3 months and for 4 month olds it's time for serious sleep training.
Self-calming skills are the key!