First, it's preschool, so I wouldn't worry about it TOO much. He will mature quite a bit even in a year. There's a reason why schools take 5 yr olds and not 4 yr olds. :o)
Second, I wouldn't feel responsible for what your son is doing at school. That is the teacher's job. And actually I would be a bit put off that they would expect me to fix something they couldn't. After all, aren't they trained and experienced? They know what to do to have it not happen.
I would talk with a 4 yr old about making choices on friends, but I don't think it will "stick" until he's more mature. It seemed like at 4 yrs, it takes things longer to change. The toddlers and preschoolers are really aggravating, but it seems in 6 months they are worlds better. Sometimes it can take my son a year or even longer until he can use a lesson we've been working on.
I don't see how taking away home privileges will help with school behavior. They seem too far removed from each other at 4 yrs old. Maybe reminding him who to hang out with and to have the teachers do the same would prove more helpful.