For cis-polyisoprene (PI) chains having dipoles parallel along their backbone, viscoelastic relaxation reflects orientational anisotropy of subchains (stress-generating units) at respective times while the dielectric relaxation reflects orientational correlation of the subchains at two separate times. This difference between viscoelastic and dielectric relaxation processes enables us to examine the short-time coherence of subchain motion in individual chains through comparison of these processes. Specifically, for the two extreme cases of perfectly coherent or incoherent subchain motion, viscoelastic moduli G(coh)* and G(incoh)* are explicitly calculated from the relaxation times tau(p) and eigenfunctions f(p) defined for a local correlation function describing fundamental features of the dielectric relaxation. On the basis of these backgrounds, the G(coh)* and G(incoh)* were calculated for a PI chain (M = 48800) dilutely blended in a high-M entangling polybutadiene (PB) matrix (M = 263000). (The tau(p) and f(p) data necessary for this calculation were obtained dielectrically in Part 1 of this series of papers.) The G(coh)* was in excellent agreement with the G* data of the PI chain while G(incoh)* was significantly different from the data, meaning that the subchain motion was highly coherent for the PI chain in the high-M PB matrix. In contrast, the subchain motion of the same PI chain was found to be incoherent in an entangling PB matrix of smaller M (=9240). The constraint release mechanism made a negligible contribution to the global dynamics of the PI chains in the high-M matrix while it dominated the dynamics in the low-M matrix. These results indicate that the constraint release is an important factor that determines the degree of coherence of the subchain motion.