This is an extremely common thing I see parents of 4-year-olds say, and my DS went through a baby-talk phase at age 4 too. We mostly ignored it, and sometimes indulged it ("Oh, we're playing baby? Fun!"). It passed within a few months. I figure he also acts like lots of other things (animals, space warriors, ballet dancers, etc.,) so why not a baby sometimes?
DH+Me 1994 DS 2004 DD 2008 DDog 2014