Personally I wouldn't game on 4K for at least another 1-2 years.
Pretty much this, since most games don't scale the UI very well for 4k displays, and you end up playing them at a lower resolution on a 4k monitor, and it looks all blocky.
If you insist on 4K gaming, then it makes no sense to get another 290, unless you can find it really, really cheap.
With the new cards coming from both nvidia & AMD, it is just better to wait a few weeks, and see how performance turns out to be.
Also, as was already mentioned, crossfire/SLI isn't automatic for most games, and you won't really see double the performance. With DX12, this could change, but, you need to wait for DX12 games to take full advantage of multi GPUs in a system.