When mine turned four she invited four or five cousins and friends to go to the Woodstock theatre to see a movie. We went to the manitee, I bought snack packs and we got out of there for less than $25. Then we went home and did a little painting craft I got from Walmart for less than $2 each and ate frozen pizza.
I made her cake and we bought icecream from Aldi.
If you wanted to do a cute little gift bag then go to the cosemtic section of Walmart and buy those 50 cent bubble gum bottle of nail polish, tiny lip glosses, little hair pretties, and some stickers or something. Even go to Family Dollar and you will find a bunch of stuff that girls that age would find really cool.
My daughters got little wooden puzzles one year that you color with tiny markers that came in a package, some tiny notepads with tiny pencils and a little coloring book with cute crayons in the shape of butterflies. I think the mom got them all from Oriental Trader and she said she barely spent $15.
Kids that age love to bake. Get break apart cookies if you don't want to let the kids make them from scratch. Then get frosting and let them decorate them to take home.
Whip together a simple little apron that they can decorate with fabric markers.
Or get those foam crowns and let the girls decorate their own crowns.
Let them make their own little pizzas with ingredients you put in little bowls in the middle of the table.
Go to Michaels and see what kind of crafts they have.
Think of a theme and go with it. Princess, baking party, dress=up party, pizza party, craft party....