OK the maga render is at a bit over 219 hours and is about 2/3 the way through, the pace has picked up so I'm hoping it might finish tomorrow.

Why do big?
Here's a couple of Blender renders I've made using the 4k version, not the project I want this environment for just some quick tests made using an abstract cityscape recycled from elsewhere. I did spend a lot of time on the procedural texturing as I needed to use Open Shading Language for one of the mix functions, I'd never even looked at that before. A bit of a waste of time as the results are a bit rubbish.
the first one is just about acceptable; the camera has a horizontal field of view or 83.267 degrees.
The background in the second one where the field of view is 35.489 degrees is just a blurred mess and these are only rendered at 720p
Admittedly I could have just rendered a background image with the required resolution but I'm not exactly sure what I'll include in the final image and have some vague ideas for animation so just want the backdrop to work hence the huge render. That will let me use a FOV of 21.09 degrees if I render at 1080p and 45 degrees at 4k.