Any Disney Park is going to work for you! They have a Ride Switch program that allows one person to stay with the baby while everyone else goes on the ride. When the group gets back, the person who stayed with the baby shows the worker their "Ride Switch Ticket" and they get to get right on the ride. That way everyone gets to enjoy the rides.
Disney has what's called theme rides, so many rides allow infants to be held. My family went 3 years ago while my nephew was only six months old. We were able to take him on alot of the rides we went on. I'll name JUST A FEW of the rides at each of the four Disney Parks that your WHOLE family will be able to ride together.
Magic Kingdom:
The Many Adventures of Winnie the Pooh
It's a Small World
Peter Pan's Flight
Hollywood Studios:
Playhouse Disney Live Show (not a ride, but the kids will love)
The Great Movie Ride
EPCOT:
Spaceship Earth (the big ball)
The Seas with Nemo and Friends
Journey Into Imagination with Figment
Animal Kingdom:
Kilimanjaro Safaris (can be bumpy, so smaller children are supposed to sit in the middle of adults)
Finding Nemo Musical & Festival of the Lion (ok, these aren't rides, but they are MUST dos)
These are just a few of the rides you can take the 4 mo. old on, but there are so many rides that your other children will absolutely LOVE!
Also, there are parades and/or fireworks at each of the parks.