  Using an inductive approach to classify multipartite entangled states under
stochastic local operations and classical communication introduced recently by
the authors [Phys. Rev. A 74, 052336 (2006)], we give the complete
classification of four-qubit entangled pure states. Apart from the expected
degenerate classes, we show that there exist eight inequivalent ways to
entangle four qubits. In this respect, permutation symmetry is taken into
account and states with a structure differing only by parameters inside a
continuous set are considered to belong to the same class.
