  The mixing between q-qbar meson and qqbar-qqbar tetra-quark states is
examined within an effective QCD Coulomb gauge Hamiltonian model. Mixing matrix
elements of the Hamiltonian are computed and then diagonalized yielding an
improved prediction for the low-lying J^{PC} = 0^{+/- +}, 1^{--} isoscalar
spectra. Mixing effects were found significant for the scalar hadrons but not
for the 1^{--} states, which is consistent with the ideal mixing of vector
mesons. A perturbative assessment of the exact QCD kernel is also reported.
