We have investigated the frequency characteristics of the coherent 420 nm beam generated via parametric four-wave mixing (FWM). A single, high-power 778 nm laser is directed through a high-density rubidium cell with a detuning of 1 THz from the intermediate state, generating fields at 420 nm and 5.23 μm through FWM. The frequency of the 420 nm light has been found to shift as the excitation laser is tuned. The measured frequency shift ratio of 1.87±0.04 corresponds with the selection of a different velocity class at each excitation frequency, implying that the 5.23 μm beam frequency is correspondingly shifted. The 420 nm light has been tuned over a range of 1 GHz. This parametric FWM process has potential application as a tunable photon source at novel wavelengths.