This paper presents the results of research conducted in the field of confinement effects in ultrathin dielectric films. Method of choice is the method of Green's functions which proved to be a very powerful tool for theoretical research in solid state physics. Among many other properties of materials, such as optical, conductive, etc. with this method it is possible to obtain critical values of boundary parameters corresponding to all transitions from bulk to localized exciton states. This research is valid for four layered crystalline film, since that type of ultrathin structure was the object of research. Conditions for occurrence of one, two, three, or even four localized states have been found and analyzed and regions with equal number of localized states have been defined. This corresponds to the process of reconstruction of the energy spectra of excitons in quasi 2D nanostructures with respect to those in corresponding bulk-structures.