Variable resolutions with fixed values

[Baylock]

I'm on a project that I eventually would like to render at 3840x2160 (with a final frame by frame export to MP4).
But during development, I would like to work at 1920x1080 as a native resolution so that I can benefit from a decent quality in terms of previews.
While working, I want to be able to switch previews from 960x440 to  1280x720 and 1920x1080.

Right now, I set the actual size of the canvas to 3840x2160 by default but resize it through css to the 3 other smaller sizes for visualisation when needed.
The result is always sharp and I like the preview flexibility it but the native high resolution makes heavy scenes a bit laggy sometimes, which is not ideal while working.
I can't go down to a native resolution of 1920x1080 because, to convert it to 3840x2160 later on, I would have to convert all my values (positions and transformations). And this is my main issue actually.

So here is my question: how can I do so that I can work at any resolution I want and easily upscale it to a better resolution later one while not having to convert my values ? This all while preserving the best rendering quality from the beginning to the end?

I think it will have to do with the resolution settings and the pixelRatio but I'm not sure how this works relative to my problem.

To summarize, I want:

Final render: 3840x 2160 (native)
Working resolution: 1920x1080 (native)
Working preview : 960x440, 1280x720 and 1920x1080 (CSS resize)
And I don't want to have to mess with the values when I set up the final rendering.

I don't need help for the css part. I just need to know how I can make sure that I can work in 1920x1080 and seemlessly upscale to 3840x 2160 later on without messing with the values.

Thanks. 

[Exca]

Create canvas with full resolution. Change the renderer resolution and canvas width/height with css.

So for example something like this:

Canvas width & height values always at 3840 x 2160.
3840 x 2160 => CSS width & height as 3840 x 2160, resolution 1
1920 x 1080 => CSS width & height as 1920 x 1080, resolution 0.5
1280 x 720 => CSS width & height as 1280 x 720, resolution 0.33
and so on.