What is a Level 4 lab?

Can you elaborate on what a Level 4 lab is? I'm curious to know if it's a specialized facility that deals with particularly sensitive or dangerous materials, or if it's just a term used to categorize a certain level of equipment or security protocols. Additionally, what industries or fields typically require the use of a Level 4 lab, and what sort of training or certifications are necessary for personnel to work in such an environment?