We provide a four-level double-V type atomic system driven by two week probe fields and two strong coupling laser beams. In the condition of resonant four-wave mixing, the two probe fields can be amplified without population inversion. Due to the fact that the two excited states are close- lying upper levels of hyperfine structure, the spontaneously generated coherence (SGC) effect must be considered. Interestingly, the amplitude of gain is sufficiently enhanced with the same parameters as those in the case without considering SGC. In addition, we find that the probe gain is sensitive to the phase of two laser fields which interact with the same lower level. To be more specific, the amplitude of gain is modulated by the phase periodically but restricted by θ (the angle between two induced dipole moments). At the same time, we also analyze the influence of the coherence pumping (strong coupling fields) detuning.