$Z_c(3900)$ as a Four-Quark State
  Using the method of QCD Sum Rules, we derive the correlator $\Pi^Z$ for a state consisting of two charm quarks and two light quarks, $c\bar{c}u\bar{d}$, and carry out a Borel transform to find $\Pi^Z(M_B)$. From this we find the solution that $M_B\simeq 3.9 \pm .2$ GeV, showing that the $Zc(3900)$ is a tetra-quark state. 
Leonard S. Kisslinger and Steven Casper
hep-ph