We report experimental studies of the degenerate four-wave mixing (DFWM) response in weak field (no saturation) and strong field (saturation) regimes for the case of nonmonochromatic, non-lorentzian fields. The saturation behavior of line-center intensities and linewidths of rovibrational transitions in the nu(3)/nu(2) + nu(4) + nu(5) Fermi dyad region of jet-cooled C2H2 are in good agreement with nondegenerate two-level saturation models, suggesting that the on-resonance DFWM response is weakly dependent on laser line shape. We find that DFWM line shapes in the weak field regime are determined by the infrared power spectrum and cubic intensity dependence, as illustrated using broadband (0.35 cm(-1)) and narrowband (0.04 cm(-1)) infrared radiation. The saturation intensity (I-sat(0)) is observed to decrease with decreasing bandwidth, in qualitative agreement with theoretical predictions.