The results of transient degenerate four-wave mixing (DFWM) and "pump-probe" spectroscopy in aggregates of 1,1’-diethyl-2,2’-cyanine (pseudoisocyanine, PIG) chloride at 300 K an reported, Spectral dispersion of DFWM efficiency within the J-band and near exciton resonance has been measured. Time response of both transient absorption changes and DFWM signal is found to be strongly dependent on pump photon fluence and wavelength. This behavior is qualitatively explained within a physical model of nonlinear optical dynamics in aggregate domains which accounts for exciton annihilation, and the effect of nonthermal phonons produced as a result of decay of two-exciton states. Intensity-dependent evolution of excess dynamic disorder due to nonthermal phonons manifests itself in pump-probe experiments as dispersion-type differential spectrum appearing at high pump intensity. The third-order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing. The nonlinear figure of merit of aggregates was evaluated which is intensity dependent due to exciton annihilation and associated subsequent processes.