Hi S.,
I haven't read the other posts, so sorry if I'm duplicating anyone else's advice... I have the same struggle with my four year old. My five year old is more cooperative, so I always unbuckle her car seat first and let her cross over on the inside to where my four year old is still buckled in. Then she (the five year old) waits while I unbuckle my four year old. When I unbuckle my four year old, I always quickly hold her hand and escort her out of the car, so that she doesn't have an opportunity to run to the back of the car (which she will every single time without fail no matter the "consequence", and bribes never work for long w/ her...) Then my five year old follows her out. I like that I have them both getting out of the car on the same side, too, it's much safer this way. I hate to think of one of them jumping out of the car alone at the grocery store or mall or something, with the crazy divers that zoom in or out of parking spaces like they're in a race...I know moms who just open the doors and trust everyone will end up where their supposed to be safely... Anyway, so that's my way to have peaceful exits from the car, and it works like a charm. I do insist on cooperation in other areas of life, but to me, this is a battle I'd personally prefer to avoid since I usually have enough on my mind when we're out anyway. And I definitely wouldn't want to start my day with this frustration...(I'm preaching to the choir, I know! :)
Hope this helps you a little!
Good luck!
K.