Si(OH)4

from The Collaborative International Dictionary of English v.0.48

Tetrabasic \Tet`ra*bas"ic\, a. [Tetra- + basic.] (Chem.)
   Capable of neutralizing four molecules of a monacid base;
   having four hydrogen atoms capable of replacement by bases;
   quadribasic; -- said of certain acids; thus, normal silicic
   acid, {Si(OH)4}, is a tetrabasic acid.
   [1913 Webster]