1080p, 900p, 720p, a dynamic frame buffer, it really doesn't matter how it's done, it just needs to happen regardless of how resolution is compromised. The game looks beautiful, it runs great at 4K/30 but knowing the hardware it's most certainly capable of this with a compromised resolution.
Now I know Microsoft is pushing "Native 4K on all first party titles" to suit their ambitions but that doesn't mean that it can't have an alternate rendering method. This would be something nice to see in the near future in coming patches, the ability to choose the experience you want.
