Very helpfull tips here. Wasted many hours and nerves trying to make my used ipod 4g work until i found this. Comunication between ipod and itunes was always interrupted. I was able to load songs at all, they were not played or only partly played. Combined the following suggestions from above and afterwards it worked perfectly!
1. opened the ipod and disconnected the cable which connects the battery with the board for some minutes
2. placed a 1x folde business card between drive and metal case, assembled ipod again
3. set ipod to volume mode by pressing Menu and selection button together until the apple sign appeared, afterwards I pressed immediately the selection and play button together until the check mark sign appeared

Now I was able to load the songs. To get back to normal mode I performed finally a reset (menu and selection button)