My son got a Leapster when he was 4 and didn't play with it much. He was bored with the games because they were too easy. My oldest daughter just turned 4 and has been playing with the Leapster for the past 8-9 months. She only plays with it on long car trips - at home and on short outings she ignores it. She's just not a video game type person.
My son LOVES the Didj (also from Leapster). Since your daughter is already 4, I wouldn't get her a Leapster, I'd spend the money on a Didj since she'll be able to play with it for a long time. It has games for 'kindergarten' (which are really more like pre-k level -- stuff kids know before kindergarten) but also has games for older elementary kids.
One thing about all those educational games... read the specifics on what they reinforce/teach because the grade levels they use are really off. My son easily plays with 3rd-4th grade level games because they covered that stuff by 1st grade!