  We investigate low-energy excitations of the one-dimensional half-filled
SU(4) Hubbard model with an attractive on-site interaction U < 0 using the
density matrix renormalization group method as well as a perturbation theory.
We find that the ground state is a charge density wave state with a long range
order. The ground state is completely incompressible since all the excitations
are gapful. The charge gap which is the same as the four-particle excitation
gap is a non-monotonic function of U, while the spin gap and others increase
with increasing |U| and have linear asymptotic behaviors.
