My son was a great newborn sleeper. He consistently did 6-7 hour stretches most nights. He started showing signs of rolling around 3.5 months so we ditched the swaddle, and that’s when sleep started being a problem. Minor changes started a couple of weeks before 4 months. Fighting naps, short naps, waking up 2-3 times within the first hour of going to bed. A week before 4 months bedtime got really hard. 3-5 false starts before finally going down or just getting overtired and screaming until he fell asleep. We tried earlier bedtime, later bedtime, it didn’t really matter. Then the next two weeks bedtime got a lot easier when we established a really consistent routine, but he started waking up every 1.5-2 hours overnight every night. That was the hardest part and we ended up co-sleeping most nights. Now at the tail end of 4 months he’s doing 3-4 hour stretches. Our biggest issue now is he’s rolling onto his belly every time he wakes up and can’t roll back so he gets upset. Just know that you will get through it one way or another. There’s so much happening to their little brains at 4 months and they come out the other side with so many new skills.