Have you read "The 90 Minute Baby Sleep Solution"? It has lots of helpful info about sleep cycles and developmental milestones and their effect on sleep and how to cope. It's a quick read too and usually cheap on amazon.
4 months is a standard "sleep regression" time, and that book can help you identify how to best help your baby. It doesn't push any one approach so it's nice in that regard. Just sleep science and some problem solving tasks.