Is it possible to do a USB miner with a BFL chip? I am guessing no since no one has done it and they are much more readily available than BF.
Might be possible, but with the BGA packaging they used makes them a lot harder to work with than the more hand-solder friendly packages like TQFP (with the leads sticking out the sides). Plus, BFL lists the chips as out of stock, so I imagine you'd have to buy them from someone who has them stocked up just like BitFury.
The BFL chips uses 3.2 W/GH/s so that's 12.8 W/chip for the 4GH/s chip. That doesn't fall within current USB specs but may be possible in the future.