This paper is the last in a series of four dealing with the problem of picosecond pulse generation in solid-state lasers. It is primarily concerned with active (as opposed to passive) modo-locking, and in particular with the early stages of the mode-locking process. The discussion is based on an aeousto-optic (loss) modulator and its interaction with the radiation present inside a laser cavity, from the moment the modulator is switched on until a fully modo-lockod pulso is formed. A brief discussion of the properties of loss modulators is alao included.The investigations have been carried out with the help of a computer model of the system, a typical Nd : YAG laser serving as an example. It is assumed that the laser radiation is composed initially of a large number of cavity modes (typically 30–100). their phases being distributed at random (narrow-bond noise). The mechanism of a gradual build-up of mode-locking under the influence of a loss modulator is carefully investigated using both the time-domain and frequency-domain (amplitude and phase) representations. It is shown that, in general, the process of active mode-locking is relatively slow, typically lasting as long as a few hundred microseconds, or tens of thousands of trips around the cavity. Furthermore, it is shown that the intensity profile of the radiation, or even its amplitude spectrum alone, fail to provide reliable information about the degree of mode-locking, the associated phase spectrum being the only safe criterion in this matter. It is also shown that, during the early stages of the modulation process, the shape of the amplitude spectrum varies in a highly irregular manner, the variations being caused by the initial lack of phase alignment among the oscillating modes. Lastly it ia shown that the phase relationships, previously based on purely theoretical considerations and claimed to be typical of a mode-locked system, in fact dovelop quite naturally as a result of the process of repeated interaction between the modes and the respective modulation side-bands. The deleterious effect of dispersion on the formation of picosecond pulses is also briefly discussed in the paper.A list of design requirements for the generation of picosecond pulses in actively mode-locked solid-state lasers is provided at the end of the paper.