Q: Is there an AM/FM tuner available by any company for a 4th generation iPod?
A: Although accessories specifically built for the 4G iPod models are becoming increasingly hard to find, the good news is that many of the accessories built for the 5G iPod that use the Dock Connector will work fine with the 4G iPod models as well.
One such accessory is the Griffin iFM Radio and Remote (iLounge ratings: A/B+), which is available in both a Dock Connector compatible version and the traditional headphone port that was used on 4G model iPods. The headphone port version will be somewhat more difficult to find, but the Dock Connector version will work fine with the older model iPods as well.
Be aware that although Apple also makes an iPod Radio Remote (iLounge rating: A-), this particular accessory will not work with 4G iPods as anything more than a remote control, as the FM radio features rely on software that is only present in the 5G iPod itself to control the FM radio tuner.
Note that these are only FM-based radio accessories. There are presently no portable iPod accessories that will provide AM radio tuning.