So, after a lot of research and a lot of running around looking at various computers I finally have a system that can play, import, edit and output 4K video. I finally realised that an old-fashioned desktop tower PC would do the job. Above are pictures of what I bought. It's an HP ENVY 700 tower, with i7 - four core - 16GB of ram and a fast 2TB hard drive. It won't edit the new Star Wars movie, but for what I need it works just fine. There are a few subjects that don't quite run totally smoothly, but there is no stuttering and my 4K footage plays a lot better than it does on my Mac's and well enough for me to see the benefit. It's a lot cheaper as well!
I'm also very impressed with my £50 software - Power Director 13. It inputs and outputs 4K, has stabilisation and image straightening as well as being simple and fast. The final movie rendered really fast, as it should with the turbo boost on the PC kicking in. It's a very capable software package and it's just what I need, Premiere and Final Cut being way over the top for what I'm trying to achieve. It also uploads very quickly to YouTube, which is a bonus. It's on sale for the next few days if you're interested. There is a free trial.
Footage was shot with my Leica D-LUX (Typ 109) and as you can see it's very good indeed. Sharp and with great colour. Again very impressive.
So all in all a good result, and the whole process has gone very smoothly, unlike my expectations. I have to say I do like Windows 8 and it seems much more user friendly than previous Windows versions I've used.
So basically it's now a case get shooting, editing and uploading. Video stock footage still hasn't taken off in a big way, so I have no expectation of instant riches. But like all stock photography, it's a long term process and eventually when the internet and our devices get faster, I'll hopefully have lots of footage up for sale. Though I must get out of my garden!!