I would do 4K 25, use the the extra frames for slow motion when needed, and render 1080P25.

Do 4k25p and render 1080p25? 
You confused me @RogerS
My project does editing on timeline and I output its signal which means the render follows my project settings. 
Or should I divide a schizophrenia for me?

Yes, edit at 4K 25p and render at either 4K 25p or 1080 25p.

I think if you crop in on the original video its better to have a project file with the same high resolution as your 4K footage. You can then render at the same or a lower resolution.

50p is for special effects in my opinion as it's smoother than what we are used to with cinema or television. I shoot 1080p 60fps for slow motion on a 24p or 30p timeline.

Rendering follows project settings using a tool like Voukoder but for MagixAVC you have to set it manually.

😁😁

I see your point!

Design a flattop and transfer it to a kayak.

With an i7-3630QM @ 2.4 GHz

😰😰

I edited 4K projects with an even worse processor than that but had to make proxy files for all the 4K footage. It worked but I'm glad I upgraded computers to be able to edit 4K easily.

4K @ 25fps and maybe smart upscale the HD footage?

Being an AI FX, does Vegas' Smart Upscale FX actually work properly with an i7-3000 series CPU? None of Vegas' AI FX worked when I was using an i7-4770 CPU powered computer.

Upscaling FHD to 4k is to my opinion a waste of time. Depending on the ratio of 2160p25 and 1080p50 footage and seen the hardware I would rather go for a 1080p50 project/render setting and try resampling 2160p25 to 1080p50 with the new optical flow project resample setting of VP19. (better results are got with the tool https://nmkd.itch.io/flowframes , which I use frequently to get smoother motion of p25 recordings). Downscaling 4k to FHD gives excellent results.

To me smoother motion is more important than the amount of pixels.

Thank you all for your replies. Yes, I know my hardware is now old and low powered but it will soon (hopefully) be replaced when finances allow, so I would be interested in what your advice would be if I had a more powerful machine. In my case, numbers of pixels may be more important than smooth motion as I have a number of very high resolution graphics and maps that I would be using along with the video. Your advice has all been most helpful.

Using the rule of lowest common factors, the most compatible and least destructive output is 1080 p25.

Or using the rule of highest common factors which would be 2160p50.

You have to experiment if the effort is worthwhile the outcome. Personally I don't see a quality difference on my 65" UHDTV between 4k and a down scale to FHD at normal viewing distance. But I see a difference at every distance between smooth motion and none smooth motion.