quaternary - noun

last 2 million years

the cardinal number that is the sum of three and one

quaternary - adjective

• consisting of or especially arranged in sets of four
• coming next after the third and just before the fifth in position or time or degree or magnitude

quaternary - thesaurus

4th   four   foursome   fourth   iv   quadruplet   quartet   quatern   quaternate   quaternion   quaternity   tetrad