The Collaborative International Dictionary of English v.0.48:

Tetra- \Tet"ra-\ [Gr. te`tra-, from te`sares, te`ttares, four.
   See Four.]
   1. A combining form or prefix signifying four, as in
      tetrabasic, tetrapetalous.
      [1913 Webster]

   2. (Chem.) A combining form (also used adjectively) denoting
      four proportional or combining parts of the substance or
      ingredient denoted by the term to which it is prefixed, as
      in tetra-chloride, tetroxide.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tetra
    n 1: brightly colored tropical freshwater fishes