Q: I recently restored my fourth-generation iPod using iTunes and now it shows a picture of an AC adapter and a wall socket. I bought an AC USB charger and plugged it in but nothing happened, so now I have an iPod that just shows that picture and can’t be used. What do I do now?
A: Until the very final set of color models, the fourth-generation iPods shipped with a FireWire-based AC adapter and cable. After completing a firmware restore or upgrade, the 4G iPod required connection to an AC based power source to complete the process. Despite the fact that these iPod models could charge over USB, for the purposes of a firmware restore they would only recognize a FireWire-based power source for this purpose. Note that FireWire and USB each supply different voltages and in fact connect to different pins in the Dock Connector port.
If you have access to the original AC adapter and FireWire cable that shipped with your iPod, simply connecting these should be sufficient to complete the firmware restore process. Failing this, you will need to find another FireWire power source to complete the process. This can be either an older iPod charger (most iPod chargers made prior to 2005 were FireWire-based), or simply an iPod FireWire cable and powered computer FireWire port, such as those found on any Mac and some desktop PCs (note that PC laptops which support FireWire frequently use a smaller unpowered 4-pin FireWire port).
If you live near an Apple Store, you could also try taking your iPod in to the Genius Bar and seeing if they can assist you, since many of them should still have iPod FireWire cables available.
Note that the final firmware update that was made available for the color-screened fourth-generation iPods (ie, iPod photo and iPod color) eliminated the need for a FireWire-based power source to complete a restore process, since these models began shipping with USB power adapters shortly before they were discontinued. As a result, if you are using a color-screened iPod, it would be a good idea to get this upgrade to the latest available firmware version to avoid running into this problem should you ever need to restore your iPod again in the future.