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Also will V4 and others from Daz still insist on installing to the same directory as Poser 8 or can I, at last, use external runtimes for them too?

For whatever idiotic reason, the people at DAZ decided it was a good idea to create an installer that absolutely requires finding a "poser.exe" file before it will install the generation4 people, so that's why you had to install it into the main Poser runtime.

However, once there, it all can be moved wherever. It doesn't have to stay in your Poser7/PoserPro/whatever version runtime. I have all my runtimes on a separate HDD and have used them all over the course of 4 different PCs over the last few years and have never had to reinstall anything, and I use the Poser Pro runtime only for Poser Pro content. None of my DAZ stuff at all is even on the same physical disk as Poser Pro, let alone in the runtime, and it all works just fine.
Python stuff, however, is probably best placed under the main Poser program runtime.

But also as others have mentioned, you can fool DAZ' idiotic installer by creating a dummy Poser.exe file anywhere you want.

IIRC , when I got M4 I installed it into a folder on my desktop that way, for the same reason I install all stuff into a temp folder first- to see what all it contains and to change folder names to suit how I want to work, not how some merchant thinks I should.