Even if your 4-month-old is sleeping through practically the whole night, it might not last. (Sorry!)
While newborns can sleep for long stretches of time, they also experience partial awakenings. And once your baby’s old enough to be aware of his surroundings, he may wake and realize that things aren’t quite the way they were when he dozed off (say, the lights are off instead of dimmed).
Steps to teach your baby to fall back to sleep, from Kim West, author of Good Night, Sleep Tight:
1. Put him down for the night when he’s drowsy but awake, even if it means unlatching him from your breast or taking the bottle from his mouth.
2. Sit next to his crib, and pat him or give him verbal reassurance if he cries (and chances are, he will). Or leave the room but check on him every 10 to 15 minutes, until he’s asleep. Only pick him up if he’s really worked up, and then put him back in his crib once he calms down. He may cry again, but usually not as much as he did before, so go back to the pat-and-reassure technique.
3. If he wakes up crying in the middle of the night, repeat the pattern, making an exception only if he’s still young enough to need a nighttime feeding. If so, try to put him back down while he’s still awake.
Remain consistent, and within seven to ten days, your baby — and you — should be sleeping through the night again.