I have purchased "5"s from Cowboom before, and they have been perfectly fine (maybe a slight scuff or scratch on the edge of the device) but I don't think I would take my chances with a "4." Still, the device is guaranteed to work, and cowboom has a very robust 30-day return policy if you are dissatisfied with the condition. I have used the return process myself, and it was very easy.
All that being said, if you are willing to go the auction route on the cowboom site, you can beat the $39 price, or even get a good deal on one of the touch models.