  The fluorescence light emitted by a 4-level system in $J=1/2$ to $J=1/2$
configuration driven by a monochromatic laser field and in an external magnetic
field is studied. We show that the spectrum of resonance fluorescence emitted
on the $\pi$ transitions shows a signature of spontaneously generated
interference effects. The degree of interference in the fluorescence spectrum
can be controlled by means of the external magnetic field, provided that the
Land\'e g-factors of the excited and the ground state doublet are different.
For a suitably chosen magnetic field strength, the relative weight of the
Rayleigh line can be completely suppressed, even for low intensities of the
coherent driving field. The incoherent fluorescence spectrum emitted on the
$\pi$ transitions exhibits a very narrow peak whose width and weight depends on
the magnetic field strength. We demonstrate that the spectrum of resonance
fluorescence emitted on the $\sigma$ transitions show an indirect signature of
interference. A measurement of the relative peak heights in the spectrum from
the $\sigma$ transitions allows to determine the branching ratio of the
spontaneous decay of each excited state into the $\sigma$ channel.
