My LO just turned 4 last week and she is wearing an XS (4-5) from Target, but from other stores I go straight for the size 5. With some pajamas I think she has a couple in a size 6. The 2 piece character pajamas seem to really shrink. She's actually a very petite girl so I'm surprised she's in a size 5 already!
What size does your 4 year old wear?