A good variety of story CDs; lots of different types/voices to keep them engaged. Alan Bennett reading Winnie the Pooh, Stephen Fry reading Paddington would be good for the 4 & 3 yr old. Music/nursery rhyme CDs too. Lots of small snacks - healthy and treats. Make sure you have planned where you're going to stop for meals and make sure they get time for a bit of exercise at those stops.
If my DS is anything to go by, they will sleep more than they usually do during the day when they're in the car.
For the older ones, get those car tidies for the back of the front seats and fill them with little toys and teddies that they can reach themselves.
In car dvd's are a real help too, we very occasionally go to dh's family, about 500 miles away and bought one for this reason. It gives them something else to do and keeps them quiet for a couple of hours. (dd1 3yo loves them and even dd2 1yo will watch them)
Have you got a LeapPad? There are lots of different types by My 1st LeapPad is designed for children aged 3 - 5 and there are loads of different games to choose from.
My 4 year old has been using his during long car journeys for over a year now and he loves it.
Woolworths, ELC, Mothercare and Argos all stock the toy (RRP about £12) and the games are sold separately. But, given the guaranteed peace and quiet it affords you on a lengthy car journey, I'd say it's worth the investment