There's no requirement to have him in preschool at that age, so if it starts to become a problem, the easiest solution would be to pull him out. Three or four years of preschool seems like a pretty strange idea to me, especially if he's already cognitively ready for K.

But yeah, what Polly said. Each kid is different, and so is each situation. If he finds enough other things that interest him, he should be fine. If not, there's no predicting how he'll respond.