To get all that AK has to offer, it definitely deserves the better part of a full day.
Three rides are absolute musts: EE, Kilimanjaro Safari and Kali River Rapids.
You should do the Maharajah Jungle Trek as well...the tigers, giant fruit bats, etc. It's very cool.
Tough to be a Bug and Wonders of Flight are also well worth it.
There some other things to take in as well, nevermind just walking around enjoying the sites so that you don't have to feel rushed.
I just can't see getting all in before noon.
Also, since it's the park that closes the earliest of the 4, if you're gonna have an evening where you eat at a resort restaurant, AK day is perfectly made for it.