The Zr4+ tetramer, [Zr-4(OH)(8)(H2O)(16)](8+), is thought to be the major component of the Zr4+ polymer system in aqueous solution, present as a dominant ionic cluster species compared to other Zr4+ clusters under various experimental conditions. Despite widespread applications of zirconium, the structure and dynamics of the tetramer in aqueous solution are not well understood. We conducted a combination of ab initio molecular dynamics and quantum mechanical studies in the gas phase and aqueous solution and related our results to the available experimental data to provide atom-level information on the behavior of this species in aqueous solution. (-)Our simulations indicate that the tetramer structure is stable on the picosecond time scale in an aqueous environment and that it is of a planar form, comprising eight-coordinated Zr4+ ions with an antiprism/ irregular dodecahedron ligand arrangement. In combination with our studies of Zr4+ dimer and trimer clusters, our results provide detailed geometrical information on structural motifs for building zirconium polymers and suggest a possible polymerization path.