  Considering quantum chromodynamics in the consistent limit of simultaneous
correlated increase of the number of colours, $N_{\rm c}$, beyond bounds and
decrease of the strong coupling to zero allows for solid statements about
qualitative features of the class of "truly exotic" tetraquark mesons carrying
four mutually distinct quark flavours. Consistency criteria extracted from
correlation functions for two-ordinary-meson scattering suggest the existence
of more than one such tetraquarks, at least, of two tetraquarks of identical
quark-flavour content and large-$N_{\rm c}$ behaviour of the total decay widths
but differing in, and hence discriminable by, their predominant decay modes
into two conventional mesons. This pairwise appearance is in conflict with the
unique variant of such a four-quark bound state arising from the binding of
diquark and antidiquark to a tetraquark by the strong interactions.
