Don't even look at that age classification thing. It is unimportant. A guest's ability to participate in a WDW ride is completely based on height, not age.
If you look at this page of travel tips for people with small children found
HERE,
you
will see a button on the right hand side of the page that says
"Height Requirements." Clicking on this link will bring you to handy
PDF that you can download. The PDF lists the attractions in all four
theme parks and both water parks that have height requirements. If an
attraction is not listed on this document it means that there is no
height requirement and any age child can enjoy the ride. The height
requirements are also listed on all the maps at the parks and on
signage in front of the various attractions. As you can see on the download, the list of rides that are off-limits
for preschoolers is very small.
All that being said, there is an element of personal judgement involved. Not every ride is appropriate for every child that meets the height requirement. For example, the Tower of Terror at Disney's Hollywood Studios and Dinosaur at the Animal Kingdom both have a fairly low height requirement - just 40 inches. There are a number of tall four year olds who might be able to ride these attractions. In situations like this, you have to think about your own child's sensibilities.
I happen to have twins. One of them begged to go on the Tower of Terror the minute she was tall enough to do so. She went on the ride and LOVED it. It's now YEARS later and her twin sister refuses to ride Tower of Terror because she is still too scared. Only you can decide where your son falls on this spectrum.