T.,
Wow, lots of good advice. For many children toilet training is a "control"issue. They can control food intake and toileting, and that is about it. As a parenting education specialist, I recommend you put her in cotton panties, with old fashioned rubber pants over them. (I like the idea of putting cloth diapers on the car seat to soak up any spills.)Children need to feel the difference between being dry and being wet. She is four and should have the ability to tell you when she has to go. Some may think this is a hard line approach, but often times as parents we want to make our children as comfortable as possible and it does not teach them responsibility for their actions. If you begin at four it will be A LOT easier when they are teens!
A sample discussion: "oh, you wet yourself. I wonder how that happened?" 'I wonder what would happen if you told me you had to go potty, what do you think?" Children need to think for themselves and learn that they are responsible for their actions. Yes, there will always be accidents, but at age four, she is developmentally ready to be dry on a car ride, unless her doctor has discovered a problem.
Good luck & be well,
E. S.