1. Denoting a chemical compound containing four elements; NaHSO4. 2. Fourth in a series. 3. Relating to organic compounds in which some central atom is attached to four functional groups; applied to the usually trivalent nitrogen in its “onium” state, R4N+, “quaternary nitrogen.” 4. Referring to a level of structure of macromolecules in which more than one biopolymer is present.