DVD player!
Stop for lunch at a place with a play area. Eat while you watch your kids play, then take their lunch in the car. Works out the wiggles during the stop, then gives them something to do during the ride.
Did I mention a DVD player?
Window clings, pipe cleaners, magnetic play boards (for your four year old)
We play car games - 20 questions (especially with Disney characters, although we do animals and vegetables too), Herbie Herbie (first person to see a VW Beetle gets a point, two if it is yellow - my DD4 loves this, but your 2 year old probably won't get it). You could also play "School Bus" or "cow" or whatever variation works for you.
Music you can sing to - whatever they like to listen to.
Plan for two stops, maybe three. Bring a ball or frisbee if you stop at a rest area.
We usually break up the trip on the way down with an overnight stop (five hours, stop for the night, five more hours), but we drive straight through on the way back.
Some people recommend leaving super early so your kids will sleep for the first few hours. We tried that once and had one of our kids up from 3 am - she never would go back to sleep. Her little sister slept, though.
Did I say DVD player? Buy a movie or two that they haven't seen before and that they are excited about.
Have a great trip!