Enterprise Architecture (EA) requires modeling enterprises across multiple levels (from markets down to IT systems) i.e. modeling hierarchical systems. Our goal is to build a Computer Aided Design (CAD) tool for EA. To be able to build this CAD tool, we need an ontology that can be used to describe hierarchical systems. The Reference Model of Open Distributed Processing (RM-ODP) was originally defined for describing IT systems and their environment. RM-ODP can also be suited to general, hierarchical, system modeling and, hence, can be used to model enterprises. In this paper, we first give an overview of our CAD tool and we present then how Part 2 and Part3 of RM-ODP were integrated to define a computer-interpretable ontology that is used in the CAD tool. This ontology is formalized using the Alloy declarative language. Last, we illustrate how the CAD tool can render Unified Modeling Language (UML) diagrams by showing selected aspects of the hierarchical systems.