qua·ter·ni·ty /kwəˈtɝnəti, kwɑ-/
  四個一組,四人一組,四位一體
  Qua·ter·ni·ty n.
  1. The number four. [Obs.]
  2. The union of four in one, as of four persons; -- analogous to the theological term trinity.
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  quaternity
       n : the cardinal number that is the sum of three and one [syn: four,
            4, IV, tetrad, quatern, quaternion, quaternary,
            quartet, quadruplet, foursome, Little Joe]