  Inelastic neutron scattering is used to investigate magnetic excitations in
the quasi-one-dimensional quantum spin-liquid system Cu2Cl4 D8C4SO2. Contrary
to previously conjectured models that relied on bond-alternating nearest
neighbor interactions in the spin chains, the dominant interactions are
actually next-nearest-neighbor in-chain antiferromagnetic couplings. The
appropriate Heisenberg Hamiltonian is equivalent to that of a S = 1/2 4-leg
spin-tube with almost perfect one dimensionality and no bond alternation. A
partial geometric frustration of rung interactions induces a small
incommensurability of short-range spin correlations.
