Things I used to do:
+Driving at night or into the night,
+stopping frequently, like every 2 hours, and then making the kids run around, chase a ball, really make them have good exercise and a work-out, get them out of breath, then they are likely to be happy to sit still for a while and possibly take a little nap (and it helps you too, as the driver, to be active and running around and having a real break from the sitting behind the wheel; just don't take a nap yourself after you get back into the car ;-) )
+story and song tapes (or CDs) from the library;
+a big bag of little toys, paper and crayons, etc, that is a travel bag only, meaning those items are not available at home;
+including some books from the library (new and interesting) and a few favorites (familiar and comforting).
+I bet the four-year old could "read" some of those familiar books to the younger sibling...
+playing "I spy" games with them, guessing games, those can be adjusted for the ages you are dealing with
+making up a long story where each person in the car gets to contribute a sentence or two in turn
+singing songs
+making up songs
+telling them old stories about family members you will be seeing soon
+ count how many red (white, blue, green...) cars you see
+count how many trucks you see
+the older one may be able to spy license plates from different states and keep track of those, and if he is excited enough the younger one will be excited and distracted, too
+draw a rough map of your route from point A to B, with some landmarks every so often (be that rest stops, cities, highway intersections, whatever makes sense) and then the older one can mark them off when you hit those (you may need to remind or help them recognize them), and that way both will have a visual on how you are progressing towards the final destination
I found the most important thing was to mix things up, to not rely on one way for too long or exclusively, to keep it interesting, and to break the journey into segments with the stretch breaks, snacks and the map, if you use that.