Radio Disney on XM. Put the speakers on in the back. Tolerate it as long as you can!
For 4 year old, I would be more in favor of Kids Place Live than Radio Disney, especially outside of the morning block.
We have moved from coast to coast FIVE times, so I know a bit about long drives with the kiddos (they aged from 5 months to 6 years during that span)
One of my best tips for a long trip is to hit up the thrift store and get a couple of "goodie bags." Most thrift stores will sell misc. small toys in a baggie for a dollar or two- old McDonalds toys and random stuff like that. Grab a few of those, it really breaks up the boredom! And when you get home, just sweep them into a bag and back to the thrift store they go!
I second stopping at Cracker Barrell, just to sit down and have a real meal, and then let the kids pick a small something from the store. Again, just to break up the boredom!
Do they have leapsters or DS or something? Keep the batteries fresh, and maybe buy a few new games (but ones that they can figure out! Nothing like trying to explain how to play a game from the drivers seat!)
Also a reminder about snacks and juices...what goes in has to come out! So control the drinks unless you are planning lots and lots of rest stops!!
To the PP who asked how to entertain a baby on a long trip...I have the best Cracker Barrell story there too! After the seconf day, my 4 month old daughter was not having it anymore, so when we stopped at Cracker Barrell, my dad bought a sparkly windchime and hung it in the back window of the minivan above her head. It was MAGICAL! and her windchime still hangs in her bedroom!!!