What do I say about why I left my last job? after 4 years as a production planner

They always ask why I left, and I freeze. There are 4 people counting on me, so I am trying to be careful. I also do not want to seem like a risk.

Thanks for writing this out. I went through something close to it 14 months ago, and it got easier once I kept a simple log of every call and form.