Perhaps the simplest fully-featured G4 application.
Based on one-file simulation by Jason Detwiler.
Installation: Compile geant4 with GDML support (and optionally HDF5 support), then do:
source (g4install_path)/share/(g4version)/geant4make/geant4make.sh
(or source (g4install_path)/share/(g4version)/geant4make/geant4make.csh)
make
Physics List: uses Geant4's named physics lists, set them using macro commands (see example run.mac)
Generator: uses Geant4's GPS. Set it up using macro commands (see example run.mac).
Geometry: uses GDML (see example run.mac). Can use materials from Geant4's NIST Material Database. Also supports Geant4's text file geometry scheme.
Output: uses Geant4's analysis manager (root, hdf5, xml, csv), with several configurable options for output format, sensitive volumes (including regex-based pattern matching / replacement), etc. (see example run.mac). Records event/track/step numbers, PIDs, positions, energies, etc.
Visualization: uses avaialable options in your G4 build (see example vis.mac).
Postprocessing: you will want to postprocess the output to apply the detector response. See example code that runs on the output of run.mac.
See similar project by Jing Liu at https://github.com/jintonic/gears