Geant4 can produce VRML, which is readable by many 3D software packages.
/vis/scene/create /vis/scene/add/logicalVolume /vis/open VRML2FILE /vis/viewer/flush
Alternately, you might only want to add certain volumes without their children. Otherwise, the VRML file could get prohibitively large. In the above command, replace the logicalVolume line with something similar to the following.
/vis/scene/add/logicalVolume HADBarrel_envelope 1 etc.
The above command adds the Hcal barrel to the scene but none of its sub-volumes.
An external VRML viewer such as Cortona is required to view the VRML output files.