I doubt that Now TV's upcoming premium box will have any influence on whether the service jumps from 720p to 1080p. We don't know if the new box will even support 4K outside of the fact the body is based around the 4K Roku we see in the US. Inside it will already be very different as it will include a FreeView terrestrial tuner.
Also, considering the existing Now TV Box is 1080p capable, Now TV could just as easily up their 720 streams to 1080p at any stage.
Anyway, I can confirm that the Apple TV app provides 720p feeds for both the live and on-demand content, for films and TV shows. This used to be different, and may still vary on other devices, but I can't check the PS4 right now as I already have reached my 4 device limit.
There was a time in the past when only the Apple TV had HD streams, but I believe that has changed.
Overall I really like Now TV. There are no contracts, and the price seems fair considering what they offer (although I think the Sports Pass is still too expensive since it doesn't have the additional sports channels).
The UI needs a bit of work. I would like to see more episode management (live we have on Netflix), and a far better Live TV experience (it is ok as it is, but it is simply not as easy as changing channels on a normal TV).
I use it mostly for on-demand content. I can't really see the point in waiting until a specific time to sit down and watch a live stream when I can just watch it via on-demand when I want, especially movies. (Although I still of course watch sport and news live).
It is a very good service that could be great if Sky allowed it to be. They are "testing" the field, as they don't want to lose traditional satellite subscribers who they charge considerably more and make more money from (especially when they lock them into long contracts). But as more and more people move across to Now TV (and they will switch), Sky should put more effort into it.