Its true that little kids love the pools at Disney and that they don't remember much of the trips (except of course the undivided attention of their parents!). But its a colorful, clean, attractive place to go so why not. My youngest was 3.5 on her first trip. The oldest was 7. They went again at 5 and 8 and then again at 7 and 10. All trips had their moments - great memories and some not so great. Its all about pacing yourself.

For a child your son's age I would recommend staying for 4 days and spend only 2 at the park. The other days you can play at the pool, rent bikes or those little speed boats, go to a character meal at one of the hotels, go to Fort Wilderness in the evening to see Chip and Dale and sing songs around a campfire. Some of the onsite hotels have fishing equipment to rent (Coronado Springs etc). Watch the fireworks. Go to the Boardwalk. You don't have to go to the theme parks to enjoy what Disney has to offer.

Darvy and others have mentioned the Unofficial Guide to WDW...get yourself a copy before you make a decision to go or not - you won't be sorry. Only you know your particular child and the Unofficial Guide takes you through each WDW ride and attraction and gives you some important information:
1. Important ride information including things that may frighten a child - such as Extreme Loudness (just hearing the word loud doesn't prepare you for the intensity of Disney LOUD), frightening features, possible motion sickness alerts, height restrictions, darkness of the attraction, etc.
2. Wait times and whether or not that wait will be in the direct sun
3. The appeal of the ride or attraction to different age groups

Disney can be magical for a 4 year old but don't go with the mindset that you have to "get your money's worth" out of that admission price. You have to do the parks in small doses with young'uns. Good luck!

We've taken our 3 kids to disney twice, the first time when they were 4, 7, and 10, the second time when they were 7, 10, and 13. We had fun both times, and the 4 yr old was fine. I would definitely not go with a child younger than 4. We did use a stroller, and we went back to the condo every afternoon for a rest. We did not stay on site (couldn't afford it), but he slept during the drive back to the condo, which helped.

The most important thing is to go when it's not too hot or crowded. The first time we pulled them out of school and went the first week of march. The weather was fantastic--warm enough to go to one of the water parks, but cool (jackets) at night and early mornings. No lines (although we used the unofficial guide to plan our schedule).

The second time we went around columbus day weekend and it was more crowded (20 minute waits) and hotter. On that trip, we stayed at the Hyatt with the nice pool (Grand cypress), and my kids wanted to leave and go back to the pool after just a couple hours of Disney.

One thing I forgot to mention about our trip when our son was four. We went in the middle of January. We walked on to everything, which made the trip even better. I would not take a child that young during the summer, nor during very busy times like Christmas, Easter or Presidents' Week.

Thanks everyone for your comments. I think on balance it'd be appropriate to wait at least 1 year. I can definitely see my son saying, "This is great, but let's go back to the pool!"

We took our daughter when she was 27 months old and she loved it!!! She rode every ride possible, and was quite a trooper. I would say it really depends on the kid, I have friends that have kids that are 27 months or around that age and would not take them to disney. My daughter is very daring and unafraid of anything. She talks about her trip to see Mickey Mouse to this day, she is now 4!!