quaternary

nouns

the cardinal number that is the sum of three and one
4, IV, Little Joe, four, foursome, quadruplet, quartet, quatern, quaternary, quaternion, quaternity, tetrad

adjectives

consisting of or especially arranged in sets of four
"quaternate leaves"; "a quaternary compound"
quaternary, quaternate
coming next after the third and just before the fifth in position or time or degree or magnitude
"the quaternary period of geologic time extends from the end of the tertiary period to the present"
4th, fourth, quaternary