OK, just to keep people up-to-date: I eventually bought the base M4 Pro. I know this this will induce scornful thoughts /comments- and I understand - the base M4 is much better value BUT, the cognoscenti seemed to suggest a 500Gb would be better in my situation (true - I have a 500GB M2 Mini which I wanted to replace/augment), but that apparently only made sense if one also bought the 24 GB version as well (for speed and reliability). Well, for an extra £350 or so I thought the MacM4Pro version was probably worth it for, perhaps, an extra year or so of use. (I decline to use the term "future proofing")! For those in the UK I bought the M4 from John Lewis who not only give a discount but also offer, for free, and extra year's warranty.
My thoughts so far: 1. Migration Assistant didn't work as well as expected and it needed several tries/workarounds before I got it to work, and, even then not all apps and passwords were transferred. Parallels and Win 11 eventually worked, but it all needed a manual install before it all worked as expected. 3. For general tasks I can see NO difference in speeds between the M2Pro and M4Pro units save Parallels/Win 11 is now better at playing, for example, a RailSim programme AND, just as important to see, a slightly more stable streaming connection between Apple Music and various Wiim devices spread around our house (thick stone walls!)
However, I do expect a (slight?) speed enhancement when using photo-enhancement software (yet to try).
In two minds whether it was worth the extra cash. I now need recommendations for some good AAA games (not necessary shoot-'em-ups to convince me I made a good investment!) Hope this helps others...