Hi,
I am not sure about the correctess of how you compute the global object position in GEANT4, but this certainly does not match with how you extract the information in ROOT. You convert to the master frame the center of the bounding box of the volume, while in G4 you convert the origin of the local reference frame for the volume. These are not the same. In TGeo, you should rather apply LocalToMaster to the (0,0,0) point to get something similar with G4.
Regards,