Why won't my turtle eat?

My aquatic turtle, Bobby, hasn't eaten in about 4 days. We've had him since August. Nothing has changed: we've cleaned him the exact same way, he has his sunlamp and he sunbathes just as often, and we feed him the same food we always have (with the occasional treat of lettuce). The only little difference is he's been shedding, but he's been growing for the last little while. We've been feeding him more often to see if he'll eat anything, but it always just ends up sitting there. He won't touch anything. What can I do to get him to eat again? Please help, I'm concerned =[