I guess it depends on your personal financial situation. To me it wouldn't be worth it, but I don't have a 4k monitor or the extra money to spend right now.
If you plan on doing 4k gaming with AAA first person shooters and such, then the extra speed (and probably more importantly more VRAM) is going to help you do that.
Note that these benchmarks are for the normal GTX cards, not the GTX Ti cards which are going to be a bit faster. If your current or expected card is a Ti, you can change it to that.http://hwbench.com/vgas/geforce-gtx-1080-vs-geforce-gtx-980