I just bought a 4k 15.6" Spectre x360. Great machine for most stuff. I'm not sure how great it would be for video editing as it does not have a dedicated graphics GPU, but I can tell you that just with regular video streaming the fan stays on all the time. It could be my particular unit. Either way, it's going back to Best Buy.
You didn't mention in your question about stylus input for your Spectre x360 but, I'll add my .02 on that as it is the reason why I'm returning the Spectre. I got the HP Active Stylus. It absolutely sucks. I cannot emphasize enough just how bad the stylus input on this machine is. The reason I bought it is to take notes in class. Forget it. Its useless. Now I know why HP is not emphasizing the stylus capability of this machine. Besides barely being able to write in any drawing/note taking apps, it's also useless for navigating about touching hyperlinks or menu buttons since you have to press so hard. Seriously. It's a joke.
Overall, the 4k monitor is absolutely gorgeous. Just be prepared for some scaling weirdness. For most things, it's fine. I use Eclipse for Python development and the toolbar ribbons in that application don't scale. They are so tiny that it renders the application unusable. There also seems to be a long lag waking up from sleep. The computers boots up from being powered off faster than it wakes from sleep mode.
I'm buying some version of a Lenovo ThinkPad. In fact, ....
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