  The models for the internal structure of the newly found four-quark
charmonium-like resonance $Z_c(3900)$ are discussed: the molecular model as
well as the hadro-charmonium and tetraquark schemes. It is argued that it would
be possible to resolve between these models by combining measurements of the
quantum numbers of the resonance and of its decay rates into yet unseen
channels $\pi \psi'$, $\pi h_c$, $\rho \eta_c$ and into pairs of heavy mesons
$D^* \bar D$ and $D \bar D^*$. The models also predict different related
four-quark states, which can be sought for in the existing and future data.
