I would love to hear more about this. I don't know that I'd be editing 4K video on my new Surface (I'm currently deciding between the Pro 4 and the Book) but you're saying the SP4 was no better when it came to 4K editing? Do you think it might've made a difference if you had 16GB SP4?
I don't think so. Video rendering requires CPU power and then ram as its two most important resources. The more cores the CPU has the better. The i7 suffers as it is only a dual core. The throttling issue makes it that much worse. The 16gb of ram certainly will make a difference over the Pro3 but I don't think it will be enough to overcome the CPU limitations.
A better solution if you require the same form factor would be the Vaio Z Canvas as it has a quad core CPU and even though it is only Intel generation 4, it runs much cooler and apparently does not exhibit the throttling issues that plague the Surface. It is also available with 16gb and has more ports than does the Pro4. I believe it is around the same price. Unfortunately it is not available up here in Canada so I have not had an opportunity to try it first hand.