As the mom of a 4 (5 in July) year old girl.... I can just say that this is, indeed, a difficult age at times. I don't remember half as many issues with my boys at the age of 4 as I have with my oldest girl.
I think that you are doing the right thing in being firm and consistent. She knows the rules, she is just testing them. And I also think switching the Leapster to AFTER rest/quiet time will help. SHe will have to earn that privilege. And if she doesn't, then no Leapster.
Hope that it gets better!