4K video editing

Lets clear up some mythology flying around here.

1. All NLEs will scale your video for you to whatever specs you choose for your project

2. Scaling video does not improve it. Ever. There's no magic here. Up-scaling 1080p to UHD (NOT 4K!), which is 3840x2160 is an even multiple. That means within a line every 1080p pixel is faithfully reproduced, and the missing pixel inbetween is created by assigning an interpolated value, and the ineropolation is simply linear. What you get is a 3840x2160 blow up image of a 1080p image, but it isn't improved, it's just a blow-up. There's no magically creating detail that didn't exist in the original. It's still just a 1080p image, and it will look like a 1080p image, though content of the image may not show that it's really a bit softer than the 4K footage.  Or it might.  The exceptions might be 3rd party apps and plugins like Topaz Video Enhance, because it does SO much more than the upscaling found in NLEs. It applies several content-aware AI based processes.  It also takes an eternity, and results vary a lot from flat out amazing to unusable.

3. Scaling video can degrade it.  If you attempt a scaling that isn't an even multiple, like 1080p to actual 4K (4096 x 2160) you'll end up degrading the 1080p image because it won't scale evently. Much more interpolation must take place, there are no even pixel relationships at all, if you want to fill the 4K frame.  Mostly when that's done it ends up just as a 2x scale and not filling the (real) 4K frame. Full up-scaling to real 4K should be avoided.

4. Down-scaling UHD/3840 x 2160 is again even math.  You can't represent the resolution of 4K at 1080p, and because it's a simple conversion, there are no real image gains here.  There will be no gain in sharpness doing that.  What you can get is the ability to crop the UHD footage without taking as much of a hit as cropping the 1080p image.  For example, if you use image stabilizing in the NLE, that process must by definition crop the image so it has room to stabilize it.  It's a good trick to shoot 4K in anticipation of stabilization in post (as is to shoot it with a higher shutter speed for stabilization).

Where you often do gain quality in down scaling is if you're output is something like a DVD, 720 x 480.  There is no even mathmatical scaling from 1080p or UHD to get to 720x480, but because that resolution is already so low, and there is so much more information in 1080p, you actually can get an apparent image improvement over shooting native 480p, particularly if the DVD is anamorphic.  That's if anyone actually does that anymore.

Since you say your end project has to be in 4K (hopefully UHD, not real 4K), then you should work in a 3860 x 2140 project and timeline.  Let the NLE up-sample the 1080p footage.  It'll be fine, and depending on the content, may not even be noticable that it's not UHD.   I agree that editing in a 1080p timeline is faster and easier, but there's no point if the final output is to be UHD.