  Coherence phenomena in four-level atomic system, cyclicly driven by three
coherent fields, are investigated thoroughly at zero and weak magnetic fields.
Each strongly interacting atomic state is converted to a triplet due to a
dynamical Stark effect. Two dark lines with a Fano-like profile are arising in
the triplet absorption spectrum with anomalous dispersions. We provide the
conditions to control the widths of the transparency windows by means of the
relative phase of the driving fields and the intensity of the microwave field,
that closes the optical system loop. The effect of the Doppler broadening on
results of the triplet absorption spectroscopy is analysed in detail.
