We discuss this a lot here -- not just the bright 3yo, but whether even an average child is ready for K. Minna has written some great posts if you want to search for them. Her dm has taught K for 35+ years. The short story is that you can supplement and tutor for academic giftedness, but you can't supplement/tutor for social skills or deficits. In other words, if your ds is not as mature as the 4k kids, it's probably not a good idea. They will be more savvy about how to manipulate him, or they might exclude him during playtime. When he's 5+, assuming the maturity difference persists, you'll see bullying. For instance, the classroom will have a behavior warning system, like a system of colored clips, and the more mature kids will know how to "play" ds and make him get himself in trouble. The older kids will probably be bigger, too.
Not knowing your ds, I wouldn't presume to tell you what he needs, but I do think you need to proceed very carefully and consider more than just academic skill, esp. if he is likely to be in K all day.
Also consider any community trends re: red-shirting. If it's popular in your community, your 4yo could be with kids up to 5y11mo, and if you don't hold him back later, he'll always be with kids up to 2 years older. That's a really big difference in maturity. You'll see a big difference now, and again in the teen years, when the older kids are ready to drive, date, or experiment with substances.
Have you read Malcolm Gladwell's Outliers? You should. It's a fast, easy read. He's a good writer. He writes about how much age and maturity affects kids' academic, athletic and social performance. He's in favor of enrolling kids on a quarterly basis, instead of yearly, so that summer babies aren't competing with winter babies, who are bigger and more mature.
Keiki's Makuahine (Keiki's Mom) 50, dh 50
Keiki: b. 2002 after 3 months bedrest
Natural conception following ZIFT/chem. pg
Olivia: b. 1999 d. 1999
28-week preemie, ptl cause unknown
Natural conception after 1 mc
ttc since 1998