The later cams don't fit into the shoe of the III. IV and later cams use the same shoe, but the IV doesn't have the zeroed groundglass.
The V and MT backs have an adjustable groundglass, so all bodies can be zeroed. I don't know if you can fit a newer back on a IV, so that it also could have a zeroable groundglass, but the back for the III is different.
In practical terms, the zeroed groundglass isn't as much of an advantage as it seems, because it's always a good idea to send the camera in anyway to have the rangefinder and groundglass calibration checked and the infinity stops and scales installed when adding a new lens to the kit. You can set the infinity stops and scales yourself, but Marflex or Linhof service can do other things like shimming lenses so that two lenses can share one set of infinity stops. For instance, a 90mm with the top rail in the retracted (-1) position can usually use the same stops as a 150mm, same for a 75mm and 135mm. Also, lenses with V/MT cams are few and far between, unless you order them new or they come with another camera body.