I had someone come to my home when dd was a similar age. It did help improve her settling off for sleeps but she didnt sleep through the night until I night weaned her while pg (heck she still wakes and comes into our bed).

4 months is known for sleep regression...have you looked that up? Even do a search here on bb.

I think they learn sleep associations around this time, so if the feed to sleep for eg they need a feed to resettle after the first cycle.

My ds has started having a dummy for day sleeps which has helped a bit, but he wakes four plus times a night and using my bed for pure survival reasons for me.

Good luck! I hope you find something that works.