Your attitude will dictate it all. And at 4, he will change and settle quickly, so that in 2 months he will be 100% focussed on the new school and new friends.
So, confident and cheerful, we are movign house, this is what new house is like, focus on his new room, and the benefits to him - look at the garden, lots of space for a trampoline etc. use photos where possible.
Then as part of that say, at the new house you will need to go to a new school. we have found a lovely one, look at this photo, they have a fish tank in reception! and so on.
If he is sad, acknowledge it, don't dismiss it, yes it is sad to say goodbye to friends, yes it is sad to have toleave. Then talk about what you could do to say goodbye, maybe make some cards with his photo/handprint and his new address to give to his friends (doesn't matter if they never write/use it, it is more as a goodbye excerise) Maybe invite all kids and parents to join you after pre-school at the play park one day. Maybe ask if you can come in one day and take lots of pictures and make a montage to take.
For most kids, getting excited about making the cards will over take the sadness.