Shipped.
Last year Nintendo shipped almost 2mil Wii U's in Q3. This year is doing better. I don't see how they can ship less than 3mil Wii U's in Q3 this year.
Sony shipped 4.5 (and sold 4.5) PS4's last Q3. This year at minimum should equal that, giving us a minimum of 18mil shipped by the end of the year.
I think Microsoft will push the XB1 and at least match last years holiday numbers. They are the real unknown in my opinion.
So if Anihawk's numbers are correct, the Wii U would have to ship about 2.8 million. I mean it's possible, but that doesn't make it likely. But with Smash coming who knows?
Like I said, 20 million for PS4 seems doable, but I wouldn't be surprised if they fell just short of it. It will at the very least hit 17-18 mil.
Xbox One... what are it's shipped numbers so far? ~7.5 million? I forget, not sure if that was ever deciphered since they combined it with 360 figures. But yeah, I guess this one is possible as well, but like you said, it's sort of a wildcard here.