  The QCD quark cyclobutadiene (ring-like), a new color structure of tetraquark
system, is proposed and studied in the flux tube model with multi-body
confinement potential. The QCD quark cyclobutadiene and other flux tube
structures of tetraquark states have similar energies and they can be regarded
as QCD isomeric compounds. The light tetraquark spectra ($u, d, s$ only) with
ring-like and diquark-antidiquark structures are calculated in the flux tube
model. The results show that many experimental states have the masses close to
the calculated values if they are taken as tetraquark states. The isotensor
states with $J^{PC}=1^{--}$ and $J^{PC}=2^{++}$ are studied and predicted that
the masses are around 1500 MeV. The multi-body interaction plays a important
role to reduce the energy of the multiquark state.
