The system of four identical bosons is studied using momentum-space equationsView original: http://arxiv.org/abs/1202.0167
for the four-particle transition operators. Positions, widths and existence
limits of universal unstable tetramers are determined with high accuracy. Their
effect on the atom-trimer and dimer-dimer scattering observables is discussed.
We show that a universal shallow tetramer intersects the atom-trimer threshold
twice leading to resonant effects in ultracold atom-trimer collisions.