This past summer I did a 2500 mile road trip (Seattle to Toronto) in a compact car (a Honda Fit) with just myself (a 25 year old female) and my German Shepherd. We drove 12-14 hours a day - but I found that I did stop more often than I would have if it was just me.
I would make my "dog stops" (for water, walks, and exercise) different from my "human stops" (food, gas, and clean toilets). I made a special point of stopping at a rest stop every 3-4 hours so that Jack could get out and stretch his legs. We also had one longer stop each day where we played frisbee for awhile in a quiet out of the way part of the rest stop. Make sure to have plenty of fresh water on hand, but I would not give food until you get to your final destination. Since you are only driving for 4 hours, I wouldn't bother making a special stop for the dog, unless he was getting really antsy and needed to "use the bushes".
As far as leaving the dog in the car, I didn't have much of a choice because it was just me. However, because it was the middle of the summer, I would crack the windows and make the stop as short as possible. I also always used the drive thru when getting my own food. I would not leave any dog alone in the car for more than a few quick minutes, especially if the dog is not used to running errands with you at home. The shock of being thrown in the car and then left alone for any length of time would stress most dogs. However, if your dog is used to coming along for trips and being left in the car while you run into the store, etc. then your dog might be fine. Personally, I'm not comfortable with it, though.
I second the advice of making sure that your cell phone number is on your dog's tags and microchip information. My Jack is a typical German Shepherd and won't wander more than a few feet from me, but I still had special tags made up for him stating that he was "visiting from Seattle", my cell phone, and his vet in Seattle's phone number. These tags can be used anytime that we travel, and I feel better knowing that we have them.
Many (I might go as far as to say most!) dogs are great travelers, once they get used to it. Jack napped the entire 2500 miles, and wasn't bothered by it at all.
