I'd recommend the latter option - buying a 1080 now, and upgrading to 4k later. Unless 4k is absolutely a requirement for you, why not wait and benefit from the improved 1080 performance?
As for the 1070 and 1080 differences, we don't know too much as of now. What we do know is that the 1070 is supposedly capable of 6.5 TFLOPS - equivalent to the GTX 980ti - while the 1080 is capable of 9 TFLOPS, more powerful than a 980 SLI config.
Based on this alone it seems trading in the 980ti would be useless unless you planned to get a GTX 1080. Though it is still incredibly impressive that the 1070 will be introduced at such a low price bracket (relative to it's performance)....