The wide-angle frames of comet C/2006 M4 (SWAN) through narrow-band CO+ and continuum filters were obtained. The brightness distribution in the plasma tail was derived shortly after the outburst of the comet on 26 October 2006. The photometric profiles were compared with the brightness distribution calculated on the basis of the diffusion model of the plasma tail. The parameters of the diffusion model were calculated to fit the distribution. The lifetime of the ions, the magnetic field induction and the coefficients of diffusion of the ions were estimated. The possible association between the change of physical characteristics of the plasma tail and the change of DE-like structures in the tail is discussed.