@hodg92,
What a great dad! I like to say that first I am so glad she has such a level headed dad.
It seems a good distance for a 4 year old to get to Oklahoma. How would she get there? You could say just like you did here - it is really unrealistic for her to visit you in OK. I have no way of getting her there and she is too young (obviously) to travel by herself. Quite honestly I agree - that I wouldn't want her to be with her for long periods of time. If she says something along the lines she would come get her and she can stay with me the entire summer - then I suggest like you said her - about her school/day care situation and how well she is getting along. Another thing is - if you take her out of the school you would still have to pay the school or otherwise lose her spot - there is no guarantee she will have a spot later on. All those things are true and work in your favor.
The one thing being so difficult for her to get there is a good reason to not have her stay with her mom. Could she come to PA to visit? Does she have family near you? That might be a good compromise and limit the time. I understand that young kids can have difficulty with changes in routine. But if her mom were to visit for say a week -- then it shouldn't hurt her too bad. And if she were local she could still stay with you and visit her mom.
Also another suggested to stay in touch - set up Skype and have her talk with her mom that way maybe a few times a week.