Well, congrats on your first trip with your son!
The first thing you should realize....you can't see it all, even in more days! LOL!!

Seriously, I would relax and try to focus on things you will enjoy and that will make the most of your trip.
Definitely spend a day at MK. There is so much to see and do and if you block off a whole day to go there, you can go slower and take in the magic!
I also recommend going to AK. AK is great for kids who like animals....the Lion King show is great and the animals and grounds are great. Don't miss Rafiki's Conservation Station..your son can meet some great animals! You can go to AK early in the morning and only spend a 1/2 day.
While EPCOT is a wonderful park, it caters more to teens and adults IMO. If your son likes fireworks, you could go there in the evening and check out some things and watch Illuminations...it is fabulous.
As for Disney MGM Studios- you could spend time here, go to Playhouse Disney, Hone yI shrunk the kids play area, etc. and take in the atmosphere of the park. Again, you don't need to spend a whole day here with a 4yr old IMO.
Take breaks in the days if it is hot...and realize that he will get tired...and that it is OKAY....you don't need to see it all to feel the magic at WDW.
If you can, may I suggest going to a character meal. This saves time instead of standing in lines to see characters and get photos. Chef Mickeys is great for character interaction...and for Pooh and friends, the Crystal Palace at MK serve breakfast, lunch, and dinner. Nice break from a hot day.
Hope this has helped a little. Enjoy yourself and enjoy the magic with your son. WDW is fabulous....just take it all in!

If you have questions, just ask!
Kamy