Resonant degenerate four-wave mixing is presented as an unusually sensitive nonlinear laser method that yields Doppler-free spectral resolution at trace-concentration levels even when using low laser power levels. Using a non-planar four-wave mixing optical setup, one can extract the signal beam from the input beams more easily, and hence, suppress the background noise more effectively and improve signal-to- noise ratios. Optical alignment is simple and convenient for this multiphoton setup. Sub-Doppler spectral resolution allows reliable measurement of many isotope and hyperfine lines using room-pressure flame atomizers, low-pressure discharge atomizers or room-pressure graphite furnace atomizers. While maintaining sub-Doppler spectral resolution, four-wave mixing still yields parts-per-trillion level detection sensitivity using these popular analytical atomizers. While flame atomizers offer convenient and fast sample introduction, low- pressure discharge atomizers offer better spectral resolution (i.e., sub-Doppler plus sub-Lorentzian), and graphite furnace atomizers yield lower atomizer background noise. Since laser power requirements are low (e.g., mW for CW lasers and nJ for pulsed lasers), many compact lasers (e.g., solid-state lasers) could be used in this simple yet sensitive nonlinear laser method.