It does exist, but it's bloody useless. Not worth arguing with him about it. It's more of a novel promo thing that gigabyte did a while back...
Hmm, I didn't know that, it's more of a concept model though right? I highly doubt the general public can get their hands on one. Besides, 2 cards in SLI really doesn't improve performance that much, I'd imagine 4 cards in SLI would offer very little bang for the buck. It would be one very expensive computer...

Find me the crossfire editions of ATI cards (or even MoBos) though
Tom's hardware did an article about it, you go find it...