From Apple's website:
AirPlay Mirroring is available with iPad 2 or later; iPhone 4S; iPhone 5; iPod touch (5th generation); and iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer) with Mountain Lion.
The iPod Touch 4th generation does not support Airplay Mirroring. Sorry.
You cannot mirror your iPod but you can use basic airplay. Mirroring will send exactly what is on your device to your TV whereas basic airplay is simply sending the video.
Not all apps are enabled for airplay and in those cases you will only receive audio.