when i had this problem i just unhook the power connector while cgminer was on and reconnected it and it started hashing. i had 2 boards like that also without the usb.
Man! That was the right hint. Somehow I never tried to power cycle the board WHILE cgminer was running... All 4 boards are running now.
You deserve my bounty of 30$ in BTC. Wanna send me an address?
rgds,
dego