DVC, or package?
On a package, you have to list your four year old as 10, and that means an adult ticket---and biometrics. I'm not sure that a 4 year old's fingers will work o the scanner, but they might.
On DVC, no worries---just list as 10, and be done with it.
One question though: why do you want to do this? Most Child TS meals should be fine for your average four year old. If CS meals are the problem, it would be cheaper to just buy the infrequent adult CS meal out of pocket than to upgrade the child.