Hi Peter,
Welcome to the Moms Panel, I hope you'll find it a valuable resource for planning your first Walt Disney World vacation!
Technically you will be considered a party of five for accommodation purposes, any child under 3 is considered an infant and does not count towards the room occupancy limits. So you could choose a room that accommodates five, as many of the deluxe resorts do, but personally I think it would be a bit cramped. We stayed at the Polynesian with our family of five and felt somewhat crowded and were lacking closet/drawer space.
You have a few other options, but a lot will depend on your budget. Any of the resorts will offer connecting rooms, so you could always book two rooms. If you were going to opt for this, I recommend my favorite moderate resort, Coronado Springs. We had connecting rooms last November and found it very comfortable for our family.
Another great option is to check out the Deluxe Villas. They offer two bedroom vacation villas that I think anyone would find very accommodating. The best part about the villas is that you will have two bedrooms, as well as a full kitchen, living room and washer/dryer. Most people don't like to do laundry or cook while on vacation, but we find it a great way to pack light and offset the cost by eating a meal a day in the villa.
To help you make your decision, I suggest you check out my favorite resource for viewing photographs of all the Walt Disney World Resorts
www.allearsnet.com. This website is a a great place to compare all of the different resorts using their detailed photographs of the rooms and resort amenities.
I hope this helps!