I started working at a daycare about 8 weeks ago. We have one child who is always in trouble. We have two other boys who have started mimicking his behavior. We have told the parents of the other two that we're 90% sure this is a monkey see monkey do situation and not the result of a deeper issue. The parents understand and so while we tell them that their child had some behavioral issues that day, we praise the child when they do something good...that way they're getting more attention when good, rather than when they do something they shouldn't. The parents of the other child have been told time and time again that their child has behavioral issues, and we've recommended some sort of evaluation for it.
If your daycare providers aren't giving you that kind of feedback, then there is an issue with the daycare. Ultimately it comes down to this, if you feel the only issue is this child being a bad influence on your child, then trust that the daycare and his teachers are working to rectify the situation.
Good luck!
ETA: I wanted to address the age difference. In my daycare, there are times when a teacher has more kids then she should, so they move a child who is about to transition up to the next class. That class may have a child who is in the process of transitioning up as well. I know there have been times when I have a 22 month old (1 year) and a three year old in the same class. During those times, it is my job to make sure that the 1 year old doesn't get run over by the older kids, and still follows the rules. It's a common practice. I would talk to the director though and ask when the 4-year old will be moved up, or if there are plans to keep your child in there regularly.