WordNet sense 1 (last 2 million years):

Age of Man, Quaternary period

WordNet sense 2 (the cardinal number that is the sum of three and one):

four, foursome, quadruplet, quartet, IV, quatern, quaternion, quaternity, tetrad, Little Joe

WordNet sense 3 (consisting of or especially arranged in sets of four):

quaternate

WordNet sense 4 (having or involving or consisting of more than one part or entity or individual):

multiple

WordNet sense 5 (coming next after the third and just before the fifth in position or time or degree or magnitude):

fourth

WordNet sense 6 (being or denoting a numerical order in a series):

ordinal