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Scaling for SLI beyond 2 cards is rather poor atm yes (hopefully it's something that will be fixed soon by Nvidia, though beyond 2 cards SLI scaling issue has existed for quite a while, it's just not brought up until the recent launch of the 900 series) ...crossfire on the other hand ain't perfect, but its scale is better than SLI for 3 cards by quite a margin (and stutter less as well) according to reviews.
4K is definitely playable with dual 290's. You'll be able to run ultra settings and keep fps high, though you may need to lower AA.
The highest temp you'll see is 94C and if the card continues to heat up to the point 95c (even though you'll never actually see this temp) the core clock and voltage level will throttle or drop lower to keep the temp locked at 94c. This is how the target temp and max fan speed work in the Catalyst Control Centre. It's actually a really clever idea when you think about it, the only let down is the cooler itself is a bit dated now. When you have 4 290X cards at 4K it's nice to see a bit of throttling to keep the fps down.
