I personally think 4th grade is really young for middle school. Unfortunately, as school districts are figuring out how to make do with their increased enrollment and reduced budgets, they may feel the only choice is to move 4th graders out of elementary school and into middle school.
If you have a 4th grader going to middle school, I'd make sure you spend time with the principal and teacher to understand how much time they'll spend with the older kids and what kind of supervision they'll have when they're together. 8th graders are talking about things that 4th graders shouldn't be exposed to!
It will also be important to talk to your 4th grader about the older kids and how to handle them.