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I assume that it is just a matter of the cutout thickness of the opening for the iPod in the dock. That said, using it for a 4G iPod would be rough on the connector if it were supporting the whole iPod instead of the dock holding it.
 
I would think so only because the 4g ipod are only 1mm (or something like that) smaller then the 3g, so it will prob fit but might be a little loose..... but don't quote me on this!!
 
Yes they fit with the 3g=>4g 20gb, but not sure on the 40gn since they are thicker, and my dad has the 4g 20gb model.
 
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