early 15c., "consisting of four parts," from Latin quaternarius "of four each, containing four," from quaterni "four each, by fours," from quater "four times," related to quattuor "four" (from PIE root *kwetwer- "four"). Also as a noun, "the number four" (mid-15c.), from Latin quaternarius.
In geological sense, with capital Q-, attested from 1843 in English, proposed 1829 by French geologist Jules Pierre François Stanislas Desnoyers (1800-1887) as name for "the fourth great epoch of geological time," but because it comprises only the age of man (now reckoned as the last 2.6 million years), and the other epochs are reckoned in the tens of millions of years, not all accepted it. Compare Tertiary.