What can I be doing WRONG!!!

I don't know what it is, but my sweet little angel has turned into a horrid brat! She never listens...to the point where it is dangerous! Like today, We were at REI and she had to go potty. And So did I...So when we went into the stall I let her go first...I guess that was my first mistake! So when It was my turn, she decided that it would be more fun to crawl out of the stall under the door! Even after I told her to stop 3 or 4 times! What was I supposed to do... stop mid stream so that I could stop my otherwise smart 4 year old from going under the stall door! I really don't know what to do! Because what I am doing now is obviously not working right!?!?! But what is worse now she has her 2 year old brother acting just like her! So here was go again! I need to figure out a way to get her back under control, so that I can get our lives back under control!