Four-step spatial quasi-phase-shifting technique, an analysis method for phase estimation from a single spatial carrier fringe pattern, is proposed for dynamic optical measurement. Based on the frequency-modulation form of sinusoidal function for the carrier fringe pattern, the phase demodulation formulas are derived with the intra-frame phase shift relations and the integral intensity in four segments. This proposed algorithm has robustness for white Gaussian noise by using the integral intensity for phase estimation, and has high spatial localization for local noise isolation. It can deal with the problem of frequency mismatch in the sub-fringe integration algorithm. Because the local frequency detection process is not necessary, the computation efficiency for the phase estimation is increased. Particularly, it is suitable for dynamic measurement. The simulation and experimental results demonstrate the feasibility and the validity of the proposed algorithm.