To calculate how much heat every 6990 in the setup will produce, you have to consider how much energy the computers containing them will draw from the wall. PSUs aren't 100% efficient, and all the "lost" energy gets converted into heat as well.
Calculating 65 W power consumption outside the video cards, we get a total of 1565 W for a computer holding four 6990s. If the PSU operates at 85% efficiency, the whole setup will draw approximately 1840 W from the wall, giving 460 W per video card.
It is far simpler. Computer components (including GPUs) do no "work" in the physics sense. So 100% of energy drawn from the wall will be converted to heat.
That's what I said (or at least tried to). I was merely pointing out that considering the GPU's power consumption alone isn't enough, since the remaining components require electricity as well and the PSU has to draw more power than it delivers to those components.