Error 4 is a problem with decoding the audio data (see viewtopic.php?f=6&t=862
) Typically that would mean a file that is not supported, and that may cause PlugPlayer to stop between songs.
Does the same playlist of songs stop in the same place every time? or is it different every time? What OS is your iPod running? and do you see the "play" icon in the top right of your iPod?