This is actually a feature or deficiency of the Podcast app, not specific to the 4th Generation iPod touch. When you finish a podcast, it will immediately begin playing the next one in line. Depending on your settings, it will either play from newest to oldest or oldest to newest.
Two options that I know work are:
1. Add podcast to playlist. Play playlist. Only contents of playlist will play. If you have only one podcast in the playlist, then nothing more will play after it's done.
2. Begin podcast and hit sleep timer button. Select last option to stop playing when current podcast ends. This will cause the current playing podcast to end when done. Note: This feature is present for audio podcasts only.
A third option you can try, but I have not tested:
3. If you like to listen to your podcasts in order of release date, set your playback settings to newest to oldest. Then when you play the least recent or oldest podcast, it should stop after it's done as there is no older podcast to play next. Try it and report back to let us know if it worked!
