I am a military spouse, and obviously we are moving more often than typical civilain person would be.
My son have had experiences moving 2 time in his life (( 5 years old)) and first for my daughter one years old.((Also a big big 9 years old St.bernard )) ^__^
We moved from OH to Utah imagine that how long it would take us on the road with little kid and older brother who loved his school and friends.
But life have to move on, we explain to our kid(s) upfront and tell them what would we will get and expected?
We also show him pictures where we will live,(( me my my husband love Utah , we bought a house in Layton area)) and he can make friends easy as long as he misses his friends.
Most important things I never make a big deal about moving, I make it simply and everyone will move in thier life time and how fun we can do , packing our favorite stuffs, tons that you can talk to your kid.
4 years old will understand and would not be a problem as long as you make sure that the home is where mommy and daddy are. That what's it.
Nice and smooth move..Don't forget some COOL movie in the DVD player in the car, GREAT snack and some Super duper cool toys for him too.
For a little one, we do stop often every 2 hours for her to walk and take a break.