There are more problems with multi GPU systems then I can care for, and believe me, I was once a believer.
Two 4890's, Two 5850's, and Two 570's are some of the configs i've tried.
Computer hardware is powerful in it's raw form, but in order to take advantage of that requires some very long winded drivers and game coding. In short, most of your GPU power is probrably used up just translating APi's.
This is how consoles achieve so much on so little. They have fast memory bandwidths and a closed system with a proprietary APi, it's a lot easier to make a game run smoothly on this than a computer, especially after a few years.
Having more than two cards creates a bottleneck at the bus. You'd need PCI 2.0 at x8 across all 4 slots to likely have enough bandwidth to truly take the cards somewhere, even then, software will bottleneck you a long time before. I'm not aware of any board out there that can do this natively.
The way they work:
SLi and CFX use alternate frame rendering to make the gains, that basically means, each card renders one frame each in sequence and then sends that frame to the primary card for output. This is bad by design, it takes all these cards a certain amount of time to render these frames, and then a little more to send them to the output, they never quite do it the same, one card will always be slower/faster than the next, and that basically creates micro stops or pulses in the framerate, which at least to me is completely game breaking, 60fps feels like 20. It's varies from game to game to unplayable, to barely noticeable. Either way, I'll never be using it again. It's called Microstutter and its awful.
Throw 4 cards doing this, and there's going to be a problem. Another inherent downside to SLi and CFX is, sure, you got high maximum FPS, but they don't seem to improve minimum FPS very well at all, and that's where it counts. SLi benchmarks quite often throw out minimums like 10 and 20 fps, even if it was for a second, but high averages and maximums.
They never scale perfectly, this again is driver and game related. The more cards the worse it gets. At 4, the last card probrably adds less than 5% on average. Sometimes you need to be playing at extreme resolutions to really take advantage of the extra power.
On the whole, I avoid SLi like the plague, unless I wanted to game at 2560 x 1600 or something, then you have to consider it if you want the eye candy.
One thing I'd always tell someone is: Always buy the single fastest card you can afford, NEVER buy two cards unless you're at the top.
If I had to tell you though, The 690's by far, less heat, less power, less space, less cash.