Methods have been developed for improving the signal-to-noise ratio (S/N) in the non-equally spaced pulse transmission method (NST or 2-PRT) for non-aliasing pulsed Doppler measurement. 2-PRT can be provided for the region T/Ts times the Nyquist frequency without decreasing the range when two pulse repetition times (PRT), T and T+Ts (Ts＜＜T), are known. Blind speed results from null frequency of the moving target indication (MTI) filter, however, as well as when S/N is degraded by the complex autocorrelation of correlation signal. We used the proposed 4-PRT method, applying the MTI filter based on either the respective-PRT-MTI or the sum-PRT-MTI technique to prove that the problem of blind frequency can be resolved. The S/N improvement method in 4-PRT makes it possible to increase the numbers of the addition of the difference vector between two phase differences without transmitting additional pulses, so that its S/N can be improved. In the proposed phase difference correction algorithm, the reference parameter obtained by 4-PRT is used to correct aliasing of phase difference, while Doppler frequency can be obtained with the conventional S/N. Measurement results obtained using synthesized Doppler signals and white noise show the respective PRT-MTI technique to be superior to the sum-PRT-MTI from the point of view of S/N in the calculation of 2-PRT. We further demonstrate that 4-PRT used in conjunction with the respective-PRT-MTI can remove blind frequency, and its S/N improvement method can reduce estimated error. The phase difference correction algorithm provides the Doppler frequency measured in the region 5 (T/Ts) times higher than the Nyquist frequency, but with almost the same S/N.