In this paper, we present an approach to the stress concentration analysis of an isotropic ultra-thin film coating with thickness from hundreds to a few nanometers coherently bonded to a substrate through an undulated interphase region. To capture the size dependence of the mechanical properties observed in nanostructured materials, we use Gurtin-Murdoch model in which surface and interphase domains are represented as negligibly thin layers ideally adhering to the bulk phases. This model is characterized by traction discontinuities at the surface and interface where the additional surface and interface stresses appear due to different bond lengths, angles and charge distribution of surface and interface atoms. In the case of plane strain conditions, the elasticity solution for a four-phase system is derived in the terms of the Goursat-Kolosovs complex potentials.