Refers to the old PLA (
中国人民解放军) NCO ranking system, where
士官 (shìguān, NCO) had six grades (
一级 to
六级).
四级士官 was a senior career soldier, typically with 12+ years of service. The system was reformed in 2022 into a new rank structure, so the term is now mainly historical or used in older novels and film.