I learned to read at four and so did my son. I would not worry about him being bored. Let him go to K and absorb what he needs. I did not really pay attention during the reading lessons and did not pick up on phonics (I learned to read from an elderly neighbor who used McGuffy Readers, which are from the 19th century!). I wish I had learned phonics as it would have helped. My oldest son was like many of the early readers here; he would end up helping other children. Some parents think that means that those advanced readers are not getting enough attention. I think that is not true as they are learning other lessons, important ones, like how to be helpful and respectful at the same time.
BTW, I have noticed that early reading usually results in kids that love learning in all forms, so becoming bored may not be an issue. Also, kids academic abilities do tend to even out. My son is going into 7th grade right now and some of the kids that were really late and who struggled when it came to reading are now doing extremely well academically.
I do understand your concern about providing the best environment for your particular child. It is a valid concern, but you should let him enjoy the early school environment. Also, he might pick up on your concerns (bright children are very aware of things like this). Also, try to avoid talking about "gifted-ness" in front of him as it is a lot of pressure for a kid. Let him discover who he is without having to deal with labels (even ones that are perceived as "good"). I say all of this as a person who was bussed to a gifted program starting in 4th grade, as the mother of two gifted kids, as a former teacher of gifted kids and now as a professor who teaches in an honors program.