J.,
You really need to tease out what is typical 4 year old behavior, and what is not. Being sassy and pushing your buttons is not unexpected from a 4 year old, and that you have a dicipline that she does not like just means that you need to use it more, use it often, and use it every single time she is sassy, pushy, or misbehaves. Thank your lucky stars that you have some leverage, and stick that crow bar under her every chance you get until she catches on.
I would say that if you have a child who is bringing up a specific and unreasonable fear as often as you say, that if that continues, that you seek help for her. It is not typical for something like being left in the car while you gas up to need to reassure a 4 year old 17 times a day that you won't do this. That could be a sign that she is expeirencing an anxiety issue that needs treatment. I am not sure how long it has been going on, but if it continues daily for the next month, I would consult a board certified child psychiatrist, as anxiety disorders are very treatable, and cause missery to little ones (and their Mama's!) We had one with this disorder, and treatment was night and day for her.
Good luck,
M.