We have a DD4, and we have been doing the 4-year old girl party all year! Some ideas:
Dora anything - toys, hair bows, coloring books, anything Dora
Polly Pocket (especially the Disney princess ones)
Playdoh, particularly the Dora playdoh (about $10)
dress up stuff - boas, crowns, skirts
personalized things - bags, towels, etc. - anything with a name on it!
summer toys - diving rings, bubbles, chalk, sprinkler to run through (once again Dora is there!)
books - particularly those with buttons you push to make sounds or books of a favorite character (do I hear Dora?

)
puzzles - 50-70 piece depending on the puzzle ability of the birthday girl. 100 piece if she is quite bright. The 24 piece say ages 3-7, but DD4 can practically do those with her eyes closed.
DD4 is very much into board games, but it is hard not to duplicate sometimes. She loves Dora Candyland (of course), Elefun, Balloon Lagoon (those last two are generally over $10 unless on sale).