[JesseK]

Minimum, if that. There is approximately 0.75" worth of output shaft inside the gearbox, which is NOT sufficient cantilever support for a 4" external shaft.

Notice the note about hardened carrier plates? That is because in the FRC 2007 season those plates would fail which would then cause the internal gears to bind up, even if the shaft was under minimal load. So regardless of your application I would high recommend you support the end of the shaft with some sort of mounted bearing plate.