Is there anywhere to get discounted tickets or does Disney pretty much have a grasp on all that? These tickets are darn expensive, especially if I want a two day hopper. I'd love to go to Epcot again, but I'm thinking with the 4 year old it might not be worth 3 hoppers just for that half day. I'm sure we could easily spend 2 days at Magic Kingdom and not run out of things to do, right?
www.undercovertourist.com for the tickets.Yes, you can spend 2 days there. The 4-year old will limit your ability to zip through everything and since they are 3 of you, you will end up doing the thing where you both wait in line, then one of you rides with the kid, then when you come back around, they let the other go with kid again. That eats up time.
At the Magic Kingdom, you could have a decent long lunch at the Liberty Tree Tavern which is a family style meal.
One nice thing about Disney is they have their indoor thermostats set at like 68 degrees and they have nuclear powered air conditioners. You might want to try a character breakfast at Crystal Palace too, but mit might be fully booked by now.
I guess the question is if you plan on doing just the stuff for the 4 year old, or want to ride splash mountain, Thunder mountain, etc. *******side note to readers....Space Mountain is closed for repairs from April to estimated November!!!!

As soon as you get in go fastpass one of the rides. It works on a time schdule and as soon as you get one the clock starts and you can get another after a certain amount of time. How much time depends on the ride, but if you plan on doing stuff just for the 4 year old. I would suggest getting a Fastpass for Peter Pan or Whinne the Pooh as soon as you get there as they go fast.
You should be able to do everything for the young one in one day. If you want to do the mountains and the kid stuff it will be tough.
If you think you got your fill from the magic Kingdom, Disney Studios or Epcot. Just depends on what you are most interested in. may be better for your 4 year old than Epcot.
Toystory mania is cool, make sure you fastpass that first thing when you get there. There is usually a line to even fastpass it and it's gone by lunch time. Looking at 2 hour wait if not fastpassed. They also have indiana jones, muppet vision, car stunt show, playhouse disney.
Your son will not be able to do Soarin, Mission Space or Test track. If you want to pick a park you will have more fun in, I perfer Epcot as an adult.