  We study the magnetism of a frustrated four-leg spin-1/2 ladder with
transverse periodic boundary conditions: the frustrated four-spin tube (FFST).
Using a combination of series expansion (SE), based on the continuous unitary
transformation method and density-matrix renormalization group (DMRG) we
analyze the ground-state, the one-, and the two-particle excitations in the
regime of strong rung-coupling. We find several marked differences of the FFST
with respect to standard two-leg ladders. First we show that frustration
destabilizes the spin-gap phase of the FFST which is adiabatically connected to
the limit of decoupled rung singlets, leading to a first order quantum phase
transition at finite inter-rung coupling. Second, we show that apart from the
well-know triplon branch of spin-ladders, the FFST sustains additional
elementary excitations, including a singlon, and additional triplons. Finally
we find, that in the two-particle sector the FFST exhibits collective
(anti)bound states similar to two-leg ladders, however with a different
ordering of the spin-quantum numbers. We show that frustration has significant
impact on the FFST leading to a flattening of the ground-state energy
landscape, a mass-enhancement of the excitations, and to a relative enhancement
of the (anti)binding strength. Where possible we use DMRG to benchmark the
findings from our SE calculations, showing excellent agreement.
