I have a 4th Gen Color / Photo that I formatted FAT32 and was running RockBox.
After uninstallation, it only boots if connected to firewire.
I did a restore via iTunes to make it a mac pod, and connected to external power source. It does the procedure (see progress bar) then reboots, but stalls at the lit-up apple logo. It will go through this and let you select a language if you connect it to the computer with firewire. It also needs firewire to fully reboot using menu+select. It then works fine after being disconnected, so long as it doesn't reboot or shut itself off after a period of not being used. Then it requires firewire to boot again.
I also tried uninstalling the bootloader with ipodpatcher in case something was left behind, but ipodpatched said 'no bootloader found'.
Any ideas? I'm assuming this has to do with the power it's receiving, but you'd think it'd boot when the ac adapter is connected and not just firewire. not to mention the 4th gen was designed for usb.