4th graders too cool for stickers? Hmm. You're right food rewards seem to be what many teachers resort to. It's hard coming up with inexpensive rewards too. How about "points" that lead to being in charge of some things in the classroom like taking attendance (which you do anyway but they can think they're responsible), collecting homework, being your helper with "important" matters like distributing memos or papers, taking someting to the office, leading the class to the cafeteria, things like that. Colorful pencils and erasers. Stores like Dollar Tree (those "dollar stores") here have books that cost just $1 and could be a major award. But no CANDY - because we know where that leads.
fresh fruit or dried fruit seems OK
avoid glace fruit or dried fruit coated in sugar