Four month old babies can sleep up to 15 hours a day. This includes three day naps as well. When your baby is between 3-4 months, his or her sleep pattern begins to change and that's why you might find that suddenly it's becoming difficult to put your baby to sleep. In the first three months, babies quickly slip into a deep sleep, but after that you might find that your baby keeps waking up after around a 45 minute sleep cycle. Don't worry. It's pretty normal for that age. Also remember that a 4-month old baby can stay up for around 1 hour 15 minutes before needing to sleep again.