A word of warning to everyone out there:
If you have rockbox installed and charge a 4g ipod using AC without pressing the menu key before connecting the charger to the ipod, it will brick your ipod. This has happened to me.
It will charge via usb, and it will go into disk mode, but it will not boot (goes to sad face ipod every time) and cannot be mounted in windows explorer, even while in disk mode. If I connect the usb cable while in disk mode the ipod reboots after 10 seconds and then goes to sad face ipod. This means it also cannot be restored, as windows programs cannot see it.
If anyone has any advice on how to restore the ipod I would appreciate it.
EDIT: The solution is to connect the ipod via firewire. I did this with a mac but it may work on a pc with a firewire connection. This stops the ipod from rebooting, as firewire can communicate directly with the ipod while it is on. I then restored it, then connected it to a pc and restored it again to change it to a windows formatted ipod. It is now apparently fully functional.