A single 980 is not going to give you playable rates at 4k because it simply isn't powerful enough. For less than $200 on top of the 980, you can get 970 sli, which offers dramatically more performance for the money spent (this statement assumes that, on average, your games scale 50% on the second GPU).
Simply put, I can see the reason why people go for a single 980 at 1440p or 1080p over SLI 970's, as not all games scale, but those that do, SLI 970 will, more often than not, trump a single 980. This is more important in 4k as it is more likely to be the deciding factor between playable and unplayable.
For 2k, a single 970 is more than enough juice on its own to play almost all games at very high settings. At such high fps, 980 is not...