Check REI, especially when on sale. Also check Campmor.com, although their sizes seem to run big. Actually, it's no big deal if you get them a little too big; it may take a bit longer to outgrow them! The pant legs and sleeves can be rolled up in the meantime. If they're a bit big, there's more ventilation inside. Just be sure there's enough room for the warm puffy jacket inside the rain jacket!
My son found a great rain jacket for his 12-year-old at Second Ascent in Seattle. Very slightly used. Of course with three kids, the outgrown stuff gets handed down to the next sibling. Just don't buy pink if the girl is the middle one of the three! The youngest boy (only 4 at the time) was willing, but his father most definitely wasn't!
The 4-year-old should be able to carry a small pack with her own wraps, a snack and a pint water bottle. The whole thing should be no more than 3-4 lbs. and she'll be proud to think she's carrying her own stuff!
Edited to correct name of store; Next Adventure is the one in Portland and I keep mixing them up.