  We show that with the fourpartite quantum channel used to teleport an
arbitrary two qubit state, we can construct a superdense coding protocol where
it is possible to transmit 4 bits of classical information sending only 2
qubits. Alice and Bob initially share a four qubit maximally entangled state
and by locally manipulating her two qubits Alice can generate 16 orthogonal
maximally entangled states, which are used to encode the message transmitted to
Bob. He reads the 4 bit message by a generalized Bell state measurement. A
generalized protocol in which 2N bits of classical information is transmitted
via N qubits is also presented. We also show that this four(2N-)partite channel
is equivalent to two(N) Bell states, which proves that we need two(N) Bell
states to teleport a two(N) qubit system.
