Not all of these are specific to B-days, but they are cool extras that can add some magic.
We picked up Birthday pins at each park and the reaction of the Cast Members was awesome. Make sure your son has each pin he gets prominently displayed.
Several times a day at Cinderella's Carousel they do a "Sword in the Stone" ceremony where they pick a child as "temporary ruler of the realm." The times are usually posted by the stone. No guarantee your child will be picked,it is nice little extra. I made sure my son got Merlin's autograph before the show started and had his B-day pin prominently displayed.
Monorail - Ask to sit in front, purple seat in waiting area, if there is room they will let him sit in the cab, the best ride is into Epcot at night. We were lucky enough that the Monorail Pilot offered to let him sit in the pilot seat before we departed and we have a great picture of him there.
Chef Mickey’s was amazing, as was the CM bringing us our drinks. She also let us know ahead of time when Mickey was coming out for my son’s B-day celebration so we would be ready with the cameras. She also corralled Mickey after he went by us by mistake and gently removed some children who were trying to hug Mickey during our pictures with hi (the parents were oblivious to what was going on).
Guest of honor pins can be bought at the Christmas shop in Downtown Disney. They engrave them while you wait.
Epcot-Land Food Area:
As you face the food places its way on the left hand side. They have it about 4 or 5 times a day (a schedule is by it). Get there about 10 minutes early. The cast members pick a bunch of kids, make them a chef's hat and brings them in to two tables. The kids spend about 10 minutes making cookie dough. They then bring the dough out of sight, talk to the kids and bring out fresh cookies in a bag for each of them. It's great.
If you are staying at a Disney Resort press "O" and ask for the special birthday message.
We bought Keith a Pal Mickey and he went everywhere with it
The Board Walk Bakery will bake a cake and have it delivered to your resort.
Make sure the Birthday is mentioned on all reservations for dining and that whoever is booking your trip makes sure it is noted on the reservation. Pam from Kingdom Konsultants made sure Keith's B-day was noted everywhere we went.
At Grand Floridian they do a Pirates cruise, 4 years old may be too young though.
Most of the special magic we experienced was a result of outstanding CM's noticing the Birthday pins and taking it upon themselves to spread the magic. Nothing beat the random acts of magic!
I also made sure we got the name, day, location and hometown of each CM that went out of their way to make my son smile and wrote a long letter to guest relations informing them of these amazing CMs.