I doubt there will be any significant number of 4k native games on next gen consoles, hell I think 720p games will be more common than 4k. But I think it's very likely both Orbis and Durango will include dedicated scalers capable of scaling everything to 4k just like the X360 does for 1080p output. Scaling from 1080p to 4K on 4K TVs apparently looks amazing and much better than 480p to 720/1080p as scaler algorithms have improved and there is far more base data to work with.
If the Nvidia Shield can output 1080p video you can be sure as hell that both next gen consoles will. these consoles will be designed to be around in 2020 and likely far beyond after all.
@TheVoxelman on twitter