Bound entangled ststes of four qubits in tomographic probability   representation
  The entanglement phenomenon on example of Smolin state of four qubits is discussed. This state is known as bound entangled state and the spin tomogram of the state is found in explicit form. The qubit portrait method is used the Bell inequality violation which provides another tool to prove the property of entanglement of the four-qubit state under consideration. 
Igor Traskunov, V. I. Man'ko
quant-ph