Overall, I think preschool is a good idea, especially for only children. This is a setting in which they can learn to function as a part of a larger group, and acquire behaviors which are required in school settings (like walking in line, sitting in his own spot during circle time, listening while others talk...etc). Preschool isn't just for academic skill acquisition but it will provide him with a good opportunity to maintain those skills, and even expand them to a degree. The other thing to consider is his age. In general, boys develop socially a bit more slowly than girls, and sometimes can benefit from starting school a year late, this also depends on when your school districts cutoff dates for school entry are. Given that your son is 4, I would reccomend putting him in preschool. Unless you feel strongly that his social skills haven't developed to the point where school will be useful for him.
I'm working my internship as a school psychologist and most of the behavior problem referrals I see for preschool and kindergarten boys are a simple matter of not having acquired the behaviors teachers expect of children in school, like standing and walking in line, waiting their turn to speak, staying in their spot (desk or on the carpet). Preschool is a good place for children to acquire and practice these behaviors.
On a sidenote, and perhaps more relevant to your question, my son will be 4 next november and will start preschool in august of the following year (he'll turn 5 in november of his preschool year). I'll definately not wait a year to put him in preschool.