  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 four-quark 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 ({\it i.e.\} a bound state of two
diquark clusters). The results support the identification of $\sigma(600)$,
$f_0(980)$ and $a_0(980)$ 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 $p\bar p$ enhancements, X(1835) and X(1812),
with the possible exception that X(1576) may be an "exotic" first orbital
excitation of $f_0(980)$ or $a_0(980)$.
