When vram isn't an issue@1600p, your 4 2GB gpus are going to be faster than 2 4GB cards, that's the point I was making, there are a few titles that can stop your 4 gpus in their tracks though, where they are vram limited with plenty gpu grunt left in the tank.
The easiest scenario to mention(as specific game titles just cause arguments) is Skyrim.
^
There is another example of many.
Afaik, there is conflicting reports that Skyrim can only use 3.1GB vram, then it crashes(it's a known bug but Bethseda won't fix it as they don't support modding, simply allow it) but others stating they can get it to 5GB vram usage too with Titans/6GB 7970's, whether or not the community fixed the bug idk as my time has passed with Skyrim.
I am not implying in the slightest your 690's are of no use, just stating that it can be breached under certain circumstances.
Before the 'ridiculous' type comments flood in, just as it's Kaps choice to purchase a bat cave full of gpu's, it's an individuals choice whether they mod the **** out of a title too.