Class 4 lasers are hazardous for eye exposure. They also can burn skin and materials, especially dark and/or lightweight materials at close range. They should be used with extreme care.
For visible-light lasers, Class 4 lasers' have an output power 500 milliwatts and above. There is no upper limit for Class 4 -- this is the most hazardous laser classification.
Class 4 is the same as the Roman numeral "Class IV" you may see on some lasers' labels. At this website, we primarily use the Arabic numerals, for convenience.