Good luck, if it fits then I'm getting myself the highest capacity battery I can find with that size.
I just measured my battery, it's around 52x35x5mm, and that's with me rounding up the measurements, so 4G ipod batteries should technically fit because there's still some room around the battery in the case.
EDIT: The 1200mah actually seems plausible, it's lithium polymer instead of the usual lithium ion. li-po is a better technology. But the 4g batteries I see have a 3-pin connector. Usually, 3 pins means there's a small PCB attached to the battery to prevent it from over-charging, which will have to be removed if you want to put it in the gigabeat.