  The proton-neutron 2 particles - 2 holes correlations in superfluid nuclei,
described by an effective, Josephson-type interaction ~[(P+N-)+(P-N+)], are
studied within a boson approximation. The strength of this interaction is
estimated using the symmetry energy for an even-even system of nucleons lying
on the same degenerate level. The RPA vacuum is obtained explicitly, and the
electroexcitation to isovector monopole states of oscillatory pair exchange
between the proton and neutron superfluids is briefly discussed.
