Hi Emma,
Four children, 6 and younger? That just puts you to super mom status in my book!
The great thing about Walt Disney World is that there is always something for everyone to enjoy, regardless of age. Most of the attractions are designed for entire families to enjoy together. There are a few attractions that have height restrictions for safety reasons. For a list of those, click
HERE.
Actually, if you can handle the plane ride, I say this is a perfect time to bring the kids. There's something magical about small children seeing Mickey for the first time or being enchanted by it's a small world. Although your youngest may not remember their first trip to Disney, the memories that your family makes will be precious.
If you wait until your children are older, you'll have different interest to factor in. Your older children will probably be interested in the thrill rides, and the other half of your family will have to find other activities. By going now, I think you'll find that most of the attractions your oldest child enjoys are ones that can be experienced by the entire family.
Hope this helps, and hopefully a magical vacation is in your future!