I listened to both the 4g imod and a 5.5g imod with a nice alo imod dock and neither gave me much of a first impression. I've owned the 4g regular ipod ever since they came out and have listened via the dock with an assortment of high end portable amps, and when I demo'd the imod it left me wanting. On the other hand, the pico shocked me on first listen with er-4s and even though $500 is a lot of money I instantly felt it was more than justified. Second and third listening sessions afterwards with full sized headphones confirmed those thoughts. Buy the pico. Oh by the way I compared the pico to the predator too with full sized cans and I wouldn't even consider the predator.
sound quality wise with er-4s, hd580, and k501, I would rank
predator < 4g imod < 5.5g imod < pico