The effects have been studied of increasing laser bandwidth and intensity on temperatures derived from broadband four-wave mixing (FWM) spectra. A simple model of optical dephasing arising from pumping of coupled transitions is presented and validated by experimental measurements. The model is found to predict qualitatively the effect on relative spectral intensity in broadband FWM of increasing bandwidth and input intensity. The implications for thermometry are discussed to indicate the circumstances in which accurate and precise temperatures may be derived from broadband FWM spectra.