Unitarity Quadrangles of Four Neutrino Mixing
  We present a classification of the unitarity quadrangles in the four-neutrino mixing scheme. We find that there are totally thirty-six distinct topologies among twelve different unitarity quadrangles. Concise relations are established between the areas of those unitarity quadrangles and the rephasing invariants of CP and $T$ violation. 
Wan-lei Guo and Zhi-zhong Xing