HELP! My 4 year old daughter refuses to potty train. If we put her in a pull up she will not go to her potty?

at all. As to when we make her run around with a naked bottom she has no problems going number 1 or 2 in her potty. She want's to go to preschool and I have tried explaining to her that she can't go until she goes in her potty. But yet again she still wont go if she has any type of undies on at all. I have tried rewards, punishment, even putting dora undies on her and telling her that Dora would be dirty if she went potty in her undies. Please help I am so frustrated and I have no idea what to do.