Its supported, but problem is its been shown that 3 or 4 titans or 780 ti's really don't give any more performance over 2 regardless of resolution or number of monitors run. In fact its proven that many times performance is worse for some odd reason(Even with the latest drivers). And then you have the obvious CPU bottleneck issue.............overall its basically tossing money down the toilet in terms of performance. 2 way sli is optimal overall.
At 4K, there is a huge jump from two 780TI's to three. You are correct that four shows no gain, even a loss sometimes, at 4k. The only time a Titan beats a 780TI at 4k is a single 780TI.
BTW, I'll sell my soul to get a 4 way Pro. You'd think someone would be selling them on ebay.