My current rig could count those 4 billions in 30 ms or less. That's less than usb handshake to xfer the new payload will take in average.
... so transfer 1,000 headers with different extranonces in one handshake. Or don't handshake every time. Or modify the firmware that speaks whatever mining protocol you're sending at it to do the increment-extranonce-and-recompute-the-merkle-root-thing itself.
All of this discussion is useless; even if you could convince us core developers that we need A HARD FORK RIGHT THIS VERY MINUTE! there is absolutely zero chance we could make that happen before the ASICS start shipping.
So: plan accordingly.