@twixx, too bad about the timing. It would still be worth adding hybrids for 3 & 4.
Not sure if it is worth the effort, but I'll mention the possibility. Swiftech has a 140mm radiator with integrated pump and reservoir that can be coupled with a Titan X waterblock (compatible with GTX 1080 Ti Founders Edition gpu).
As you noted, putting four gpu side by side just naturally restricts airflow. You might give some thought to using hybrid gpu. While they have a higher upfront cost, they eliminate the need for a custom loop while offering much of the benefit. Presumably the idea is to add gpu as demand increases and budget becomes available. So the added cost gets spread out over time.
At the moment an i7-6850K looks to be the optimal choice. But if time permits, it may be worth waiting a few months to see if Skylake-X offers something better.
You may be able to keep the H440. It should have enough locations for mounting two or three 120mm gpu radiators along with a 240mm cpu cooler radiator. If you get a motherboard like the Asus X99-E WS/USB 3.1, you may have to measure the interior to see if the motherboard area is wide enough to take the extra inch of the CEB form factor. (CEB uses ATX mounting points.)
You will likely need a 1500W+ psu. Pricey. Evga offers a number of excellent units in that range: 1600W P2, 1600 T2. Corsair offers the excellent AX1500i. If the system will be working as hard as suggested, a 80+ Titanium might be worth the extra cost.
