We used a travel bingo game which might be great for the 4 and 7 yo - they have to spot the items on the card, but instead of using chips, there are little sliding plastic doors that cover the spot. There are also magnetic versions. Items to discover include motorcycles, bridges, barns, cows, etc. Look in the travel games section of the toy store, or in specialty or classic toy stores.
There are hand-held games like Yahtzee (for the 7 yo) and Connect Four (for both) which actually provide electronic entertainment and brain stimulation at the same time. Math skills, pattern recognition, etc.
The 7 year old could play the license plate game - either writing down the states, or having either a checklist or even a magnetic US map to mark with magnets, every time any of you spot a license from that state. Trucks are great - they often have several plates!
Sing old style songs - I've Been Working On The Railroad, Erie Canal, Take Me Out To The Ballgame, etc. You can find tapes/CDs of these, or borrow from a large public library. My family always did this when I was a kid, and later when I became a music teacher, I found that many kids today had never heard these songs - so I did a whole unit on Americana! Your little one will enjoy the singing too. Some songs have finger play motions, which is good for boredom as well as fine motor skills. Again, try the public library for books that will teach you all the things you have forgotten from your own childhood!
A lot of bookstores (Barnes & Noble) have little kits that look like decks of cards - 52 things to do on a car trip, or 52 things to do on a rainy day (which is anytime you are cooped up). They are great activities, and they get families talking, something we do too little of these days when we're all going in different directions or catching quick meals in front of the TV.
There are also travel desks you can use in the car so the kids can color - as long as you are on major highways without too many bumps. There are also "back seat organizers" to keep the games, markers, tapes, etc. all organized so they aren't all over the car.
Don't be afraid to have some quiet time too - even with quiet music so they can all nap. Or just enjoying the scenery! Don't miss this opportunity - they don't have to be actively entertained at every moment.
Good luck, and enjoy!