The Geant4 interface implemented in the FASTRAD software provides a tool able to create the 3D geometry, define the particle source, set the physics list and create all the resulting source files in a ready-to-compile Geant4 project. The tool is useful for young engineers who need to be driven into the Geant4 world, and who can use FASTRAD as a tutorial tool, or by experts who do not want to spend time on the creation of C++ files that describe the geometry, material, and basic physics and who can use the Geant4 project created by FASTRAD as a base that can be enhanced by specific features relative to their physical application. The Geant4 interface gives the software a wide range of radiation related fields, as Geant4 is already used for space, medical, nuclear, aeronautical and military applications. Its radiation CAD capabilities facilitate the engineering process for any radiation sensitive system analysis.