The partition functions Z(T) for the dusters Ar-n, Kr-n, and Xe-n (n=2, 3, and 4) were calculated with the smoothed density of energy levels p(E). The latter was determined in the semiclassical approximation by Monte Carlo integration over the phase space and corrected by the rotational asymptotic for the lowest levels and by the trajectory separation method for the bound states above the dissociation threshold. In the temperature range of 5 < T < 150 K that is of crucial interest for the cluster formation studies in the supersonic jets, the results have an estimated accuracy of about 5%. The structure of the phase space of tetramer (n=4) clusters and their conformational transition dynamics were studied. The possibility of a link between such transitions and clusters melting is discussed.