You know your daughter's personality and what works with her better. In general I find with our child more honest information is better.
"I found out something interesting - it is actually really common for four year olds to like to play that they are younger and talk like they are younger. Being four can be a time of feeling both big and small at the same time and that can be kind of confusing to sort out. What kind of things can you do now that you are four that you couldn't do when you were a baby?... I realized that what might work well is for us to compromise a bit...." Then, practice a bit the difference between the way babies talk and using a preschooler voice (or whatever you want to call it) and say that you are going to be more open to playing baby some of the time and sometimes you are going to ask for her to use her strong voice (or whatever you are calling it). We talked about compromise with our preschooler and it was something he could understand.
For us it seems if we've put that time into having a real discussion often it makes the in the moment stuff easier. If you've already talked about it and agreed on a way to describe the voice, you'll be ready to just give her a small reminder and not get into a big discussion.
Good luck and tell yourself enthusiastic baby play will make it go away sooner. Also, this is a really obvious thing but if she doesn't already have a baby doll and accessories that might be a good purchase. You could even play twins where she's one baby and the doll is another. If the doll prop is there she may transition to the mom role sometimes too, but I wouldn't push that, it'll happen eventually.
