Z_c(3900) - what is inside?

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.