Usually a 4T, or a 4/5 is a pretty safe bet.
My DDs all stayed pretty close to their ages in sizes. Right now DD9 wears a size 9 or 10 in most things... warning rant ahead: WHY OH WHY do most little girls' sizes go 7/8 then skip to 10/12??? There is a whole world of difference between those two sizes!! DD couldn't wear a 7/8 if her wee little life depended on it, but SWIMS in a 10/12! Thank heavens for Gymboree and their dedicated size 10 clothes... no 12 involved. Ok, rant over.
DD6 is tiny, and wears a 5. Most size 6 stuff is just too big, and for warm weather she can still wear size 4 shorts (don't have to worry about length). But she is definitely small for her age, most kids her age are wearing size 6/6X.