Personally, I'd agree with a trip to Disney being something really special on its own. I know many adults who have never been! If it were me, I'd buy her first pair of mouse ears (with her name, of course!), and take/get lots of pictures. After the trip, I'd make a book for her to always remember the trip (I like to do it at Snapfish after trips with niece & nephew, so they'll have those memories). At 4, she's probably honestly not going to remember a lot of the trip (nephew was 4 when we took him the first time, niece was a baby - she LOVES looking at that first book, even 5 years later).