M.L.
I have a long-ago picture of all my children when the youngest one was a newborn. His siblings suddenly wanted to be babies. So I took a picture of all of them together - the youngest one in his bassinet and the other three drinking out of baby bottles. The phase didn't last long after that.
And I don't think this will. Four-year-olds have many interesting things in their heads. If he wants to play this game, how about playing with him for a bit? "It's too bad you're back to being a baby, because babies can't eat hamburgers" (or something else he really likes). "I wonder if we can find a diaper that will fit you." "Babies aren't big enough to play on swings, but if you want to go outside I could put you in a playpen."
This game has to be played VERY lightly and in private, though - you can't be doing this angrily or with anyone else around, or it would be teasing! When he decides that he doesn't want to be THAT much of a baby, you can have a laugh together and go do some big-boy stuff.
He's enough of a "baby" to hug, though. In another ten years, you'll call him your baby and he'll say, "Aw, MOM!"