The interaction of a four-level atom consisting of one execited state, two upper levels and one ground state with two coherent fields is considered. We use one field to drive the transition between the excited state and the ground state, and simultaneously apply another field to couple the ground state to the upper-level doublet. We find that when the dipole moments between the upper doublet and the ground state are parallel the central linewidth of resonance fluorescence spectrum of the driven transition can be much narrower than both the linewidth of spontaneous emission of the excited state and of the upper doublet due to quantum interference between two decay pathways from the upper levels to the ground state of the atom.
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