Converting a THz signal into the optical domain is of great interest for THz sensing and spectroscopy. Here intense broadband THz pulses with a central frequency of ΩTHz are mixed with an optical pump at ωp, and a signal is observed at a wavelength of ωs = 2(ωp - Δωp) - ΩTHz with the detuning ωp - Δωp being due to pump pulse spectral broadening. The observed THz-four-wave mixing (FWM) signal close to 400 nm is shown to result from a resonantly amplified four-wave mixing in CaF2, BaF2, and MgF2.