What should i get a 4 years old for her and her birthday?

My bestfriend 's sister is 4 years old,she is one of my friend too she was so excited when she heard that i was gonna buy her something. but i have no idea what to buy. She loves dora and disney princess and american girl dolls. I also have a nephew( the son of my oldest cousin) he is 5 years old and very active and violent. i am 12 so i dont have alot of money, kinda on a budget. help, what do you give someone for their birthday when your on a budget? thank you so much for readong