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Quantum systems consisting of few resonantly interacting particle are universal
in the sense that their features
are insensitive  to the short-range interaction details.
The best known example is the Efimov effect, where the large two-particle $S$-wave
scattering length $a$ leads theoretically a to high (up to infinity)
number of weakly bound trimers  \cite{efimov:plb}.
Furthermore, Refs.~\cite{hammer:07a,stecher:09a} predicted
the existence of two  tetramers for each Efimov trimer.
The two lowest tetramers are true bound states but are affected appreciably by the short-range
corrections. In contrast, all higher-lying tetramers
are unstable bound states (UBS) \cite{deltuva:10c,deltuva:11a} since they reside
above the lowest particle-trimer threshold, and can decay into two-clusters,
therefore having finite lifetime and width.
The elaborated bound-state methods \cite{hammer:07a,stecher:09a,yamashita:06a,blume:00a}
may be not capable to treat rigorously
also the four-particle continuum as needed for the study of higher tetramers.
However, in this case the technical difficulties in describing the
scattering processes involving very weakly bound dimers and trimers
in the universal regime may limit the accuracy  of the
coordinate-space methods \cite{deltuva:ef,lazauskas:he}.
Alternative calculations using the momentum-space framework for four-particle transition operators
appeared to be the most powerful one. In particular, with the choice of simple rank-one separable potential
one continouus momentum variable could be eliminated, allowing unprecedent accuracy in treating the other
two Jacobi momenta. This enabled to include a large number of trimer states with very different momentum/energy/radius scales
and to achieve the universal results with high accuracy. Examples are the atom-trimer
\cite{deltuva:10c} and dimer-dimer \cite{deltuva:11b} collisions and the four-atom recombination
\cite{deltuva:12a}. A particularly interesting feature was predicted in
Ref.~\cite{deltuva:12a}, where increasing the interparticle attraction and moving away from the
unitary limit toward larger $1/a$ the UBS of the shallow tetramer crosses the  atom-trimer
threshold and becomes an inelastic virtual state (IVS), but the close to the atom-trimer and dimer-dimer
threshold intersection  reappears again as UBS through the atom-trimer and decays again through the dimer-dimer
threshold. The conjecture is that coinciding thresholds generate an additional attraction, leading to
reappearance on the tetramer UBS.

It is an interesting question whether this effect of coinciding thresholds would manifest itself in more
complicated systems, for example, in the five-boson system. Beside all tremendous complications in the
description of the five-particle system, achieving the universal limit would become more and more difficult
even from a theoretical point of view. The only bound tetramers would be the lowest ones affected by short-range
details, thus, the atom-tetramer scattering will likely be affected as well. The only ``clean universal'' scattering
process would be that of dimer plus trimer. In principle, near the intersection of the dimer-trimer and atom-tetramer
thresholds the IVS-UBS transition could take place, but might be masked by the finite width of the tetramer.
Furthermore, due to weak binding of the shallow tetramer, the three cluster channel will open very soon
(except for the atom scattering from the ground-state tetramer), adding one more complication to the problem
with kernel singularities or nontrivial boundary conditions. Thus, the continuum of the five-boson system,
though very tempting due to its expectedly rich physics, would be much more difficult to calculate, and especially
to achieve the universal regime.

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