We learn that Cory is now in 11th grade. However, since Cory was in 6th grade in season 1, he should be in 9th grade in season 4.
Shawn: I need to know when was the last time you slept inside?
Cory: Last night.
Shawn: I can't work with you.
Shawn: Wow, and to think, you were my only rich friend.
Cory: Comfortable, never rich.
Shawn: Indoor plumbing, rich!
(As Amy and Alan are arguing)
Eric: Hello, is this obvious only to me? Streudel! (Displays a streudel)
Alan: Eric, no streudel!
Eric: Fine! Your out!
Alan: Nap time! (Points to the living room)
Eric: Yeahhhhhhh, you're right...
I'm a teacher, not a heart surgeon!
This is a reference to when William Daniels, who plays Mr. Feeny, played Dr. Mark Craig, a heart surgeon, on the TV show, St. Elsewhere