J.D.
My baby did great until the 4 month mark too. I would try to constantly give him the paci and it would sometimes work. Then I was giving him gas drops before the car ride because I thought maybe he had gas. It worked, but I also started pulling the sun shield down so he can't see out the window. That's all I do now, and no crying now. I think he was getting motion sickness. (He would stop crying when we would stop the car at a light for example.) He never threw up but I think the motion upset his stomach. I got sick in the car as a child so I don't think its so far fetched. Try blocking his view so he can't see the motion outside the car.