It is pretty normal for babies to wake in the night and "practice" new skills...and I do remember DD doing this at the same age (babbling away etc). Now that DD is in her own room, some nights I still hear hear talking to herself at that time, and most of the time she goes back to sleep, but sometimes I have to give her a quick boob to resettle.

When DS wakes at 4am, do you try and tend to him straight away, or can you leave him for 20-40mins and then give him a boob, so he's had a chance to do his thing before being resettled?

I hope you find a solution, because tbh that's the main reason we moved DD out of our room (and I still miss her!)