I'm pretty sure 4th gen can be hooked up, but not with the 300v. I think you will need the ip500 adaptor. The 300v only works with 5th gen video and nano. By the way, I have been extremely impressed with the ipod control using the 300v. Its very fast, and transfers all the metadata (artist, genre, album, album art...love the album art).
If this is the only thing the ipod will be used for, the USB function of the 7100 will read files off the ipod, using any regular ipod usb cable. You would have to load them onto the pod as folders, (not through itunes). I dont really recomend this way, because you wont be able to search by artist/album etc, only a file up, file down function. Might be good enough to pass if you are buying an ipod in the future.