Unitary Invariants and Classification of Four-Qubit States via   Negativity Fonts
  Local unitary invariance and the notion of negativity fonts are used as the principle tools to construct four qubit invariants of degree 8, 12, and 24. A degree 8 polynomial invariant that is non-zero on pure four qubit states with four-body correlations and zero on all other states, is identified. Classification of four qubit states into seven major classes, using criterion based on the nature of correlations, is discussed. 
S. Shelly Sharma and N. K. Sharma
quant-ph