I'd put him in 4's, where he would be if you had enrolled him earlier. He may or may not be exactly 'on track' with his peer group there after starting, but kids vary so much in both their 'academic' capabilities and their behavioral capabilities at that age, that you'd be better off risking that he have a bit more to learn academically to 'catch up' to the others than risk that he be bored throughout the 3s class (and again in the 4s class) and act out behaviorally from sheer boredom. And don't worry about the possibility of holding him back from enrolling in kindergarten if necessary. You'll know by then if he's ready; and if he's not, it doesn't say anything bad about him or you to make the choice to give him more time to prepare for it if he needs it.
The other possibility is to find a preschool that groups both 3 and 4 year olds together within the classroom. My son attended such a program and it was great. The academics were geared to their age/capability but could be more challenging for those who needed that, and behaviorally speaking, it gave 3s older kids to emulate and 4s someone to model good behavior for. My son is an only child, so that proved helpful in terms of socialization and preparing for being in Kindergarten when the entire rest of the school was bigger than him. He was very prepared for kindergarten academically and socially.
This approach could also help alleviate any concerns he may feel if you decide to continue another year before KG - that his friends are moving on and he is not - because some of his friends will move on and others will naturally remain in PreK. It will be easier to explain that it's "just not his time yet".
It's a tough call, but I'm sure you'll make a good choice for him, whatever it may be!