Four-quark states with different internal clusters are discussed within the constituent quark model. It is pointed out that the diquark concept is not meaningful in the construction of a tetraquark interpolating current in the QCD sum rule approach, and hence existing sum-rule studies of fourquark states are incomplete. An updated QCD sum-rule determination of the properties of diquark clusters is then used as input for the constituent quark model to obtain the masses of light 0 ++ tetraquark states (i.e. a bound state of two diquark clusters). The results support the identification of σ(600), f0(980) and a0(980) as the 0 ++ light tetraquark states, and seem to be inconsistent with the tetraquark state interpretation of the new BES observations of the near-threshold pp enhancements, X(1835) and X(1812), with the possible exception that X(1576) may be an "exotic" first orbital excitation of f0(980) or a0(980).