1) Digit 2) Figure 3) Four 4) Foursome 5) Little joe 6) Quadruplet 7) Quartet 8) Quatern 9) Quaternary 10) Quaternion 11) Tetrad 1) Four 2) Quadruplet 3) Quartet 4) Quatern 5) Quaternion 6) Tetrad 
fourness; any group of four things
• (n.) The number four. • (n.) The union of four in one, as of four persons; -- analogous to the theological term trinity.
Qua·ter'ni·ty noun [ Late Latin quaternitas , from Latin quaterni four each: confer French quaternité .] 1. The number four. [ Obsolete] Sir T. Browne. 2. The union of four in one, as of four persons; -- analogous to the theological term trinity ...
a group or set of four.