[en] We construct envelopes of holomorphy for model manifolds of order 4 and describe a class of such manifolds whose envelope is a cylindrical domain (with respect to certain variables) based on a Siegel domain of the second kind. This enables us to prove the holomorphic rigidity of model manifolds of this class. We also study the envelope of holomorphy of a special model manifold of type (1,4) and show it to be a domain of bounded type whose distinguished boundary coincides with the initial manifold. The holomorphic automorphism group of this domain coincides with that of the initial manifold. The envelope of holomorphy is fibred into orbits of this group. The generic orbits are 8-dimensional homogeneous non-spherical completely non-degenerate manifolds in C⁵