The G4Root package is an interface allowing running a GEANT4 simulation with a ROOT geometry. In fact it is just the G4 navigation that uses directly the TGeo geometry.
Since Root development version 5.27/xx the G4Root package is distributed in geant4_vmc. In spite of being provided within Geant4 VMC, it can be built and used independently from VMC.
See more details on the G4Root web page.