My wife is a Disney expert. I'll see what she can add. If you can think of any other questions you might have about the place, ask here and I'll get her to answer.
Okay, according to my husband it looks like I am the expert he spoke of. Staying on Disney property will save you time and money especially with a five year old. Do not expect to do everything disney has to offer in 4 days with a four year old.
In December in 2006 as long as you go before Dec. 20th it will be value season at Disney hotels. Rooms at value resorts during this time can be as little as $59/night with AAA discount or $69/night without. Staying on property saves you $10 day in parking fees and you can use Disney Transportation to and from the parks if your son needs a break during the day. Staying onsite allows you to use the parks during Extra Magic Hours which on select days they open the parks one hour early or stay open 3 hours later to resort guests only.
You can opt to get the disney dining plan it was going for roughly $35/per adult per day...if you choose to do this it can work out to be a bargain but you need to make reservations for sit down restaurants ahead of time because they rarely have room for walk ups. Disney restaurants are now taking reservations 180 days ahead of time.
Passes are now called my way, you buy a base ticket which allows you into one park for one day you add options onto it, like park hopping and additional days, you can also add "plus" features like the water parks and/or disney quest (most likely on a 4 day trip you wouldn't get to use this feature) you can also add a "no expiration" option which can be a good value or not depending on the number of days you choose. DO NOT BUY TICKETS ON EBAY, you do not know how many days are really left on them, and disney can't tell you until you go to put it through at the gate. With the new my way tickets the more days you buy the cheaper it is per day, so if you ever plan on returning it might not be a bad idea to buy 8 or 10 days with the no expiration option as opposed to a 4 day 14 day expiration ticket.
Ticketmania is a good discount ticket broker, we have personally used them and found their service to be exceptional.
Guide books, the unofficial guide is good but I love passporter, they also have a message board at passporter.com
www.mousesavers.com is a great resource as well as www.allearsnet.com
As far as a rental car you might want to skip it alltogether, Disney provides free transportation to and from their resorts to the airport, they provide transportation from all hotels and parks. You really don't NEED a car. If you choose to go to universal there is a LINX bus that goes (I think it is $8 per person) or you could get a cab.
Don't underestimate, the dining and entertainment options at Epcot and MGM, there is a new stunt show, there will be a new attraction Expedition Everest at Animal Kingdom, and Soarin at Epcot is not to be missed.
One last thing, at Christmas time, there are extra things, like Mickey's very merry christmas party (requires separate ticket) but it has a limited attendance so attraction lines are very small. The candlelight processional and the Osbourne lights among a billion other things.
PM me if you need more info.