Hi Heidi,
My 5 year old son absolutely loves Disney's Beach Club Resort and the Animal Kingdom Lodge.
He loves the Beach Club Resort because of
Stormalong Bay. That is the main pool shared by the Yacht and Beach Club Resorts. It's a mini-water park with a sand-bottom pool. The kids' area of the pool has a sand island in the middle. Young guests are actually out there with shovels and buckets making sand castles. When my son was there last summer, I could not get him or my cousin's 2 year old daughter out of that pool. They could have spent the entire vacation there! In my opinion, it is the best pool on property!
The Animal Kingdom Lodge is also a very nice resort. My son loves the idea of looking out the window and seeing animals! The pool is also very nice, though not as nice as Stormalong Bay. The last time we visited, it was possible to use night vision goggles to look at the animals at night. A lot of guests really enjoy doing that when it is being offered.
However, I recommend you consider staying in a Magic Kingdom (MK) Resort. With a 4 year old, I'm guessing you will be spending a lot of time in that park or in Disney's Hollywood Studios, which also has a lot to offer young guests. Staying at a MK Resort will allow you easy access to your hotel room for a nap, etc. The four MK Resorts are the Contemporary, Polynesian, Grand Floridian, and the Wilderness Lodge. My personal favorite is the Wilderness Lodge, though it is not on the monorail. However, it does offer regular boat service to and from the Magic Kingdom.
Enjoy planning your vacation and write back if you have any more questions!