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!!