It would be great if Sky Go on ATV did do 4K, but I think it's highly unlikely. It would prove just how bad the Sky pucks really are in terms of functionality and stability, plus it would stop people from switching over to Glass if they can get the same content, or in fact more content (Sky Q has more channels than Glass) via an app on a third party box.
It's more likely that Go will be HD at best and NOW will get an upgrade to offer 4K/HDR streaming with an added, more expensive Boost option. NOW also need to fix their ATV app though, as it currently only offers 5.1 audio on live content, all the on demand streaming is stuck at 2.0 stereo.
