Having just completed a 4 hour move with the girls, the one thing I recommend is stopping as frequently as you can! We had 3 stops in total, one for about an hour and these were the times when most of their munching happened.
I didn't even think about using pet remedy but the girls were just fine, I did worry as they had 5 nights in boarding at one end, followed by the drive, then 2 nights boarding at this end!
They were stars & attracted lots of attention from other travellers when we were stopped at the services, they were in a large dog crate in the boot, so the boot was wide open on our stops for them to people watch!