Not at all! Since it’s half of 9, it can therefore work with half of the software RHEL 9 is compatible with, but since the 4 is also divisible by 2, it can handle all of that software as well.
It’s really quite simple, I don’t see how this could be confusing at all…
I don’t recommend it, the more boots you use the heavier your computer will be. I’ve got mine down to half a boot and I’m working on getting it down to a quarter.
Honestly, I’d say the defaults most distros use will be fine for most users… If they don’t know why they should use one filesystem over another, then it’s almost certainly not going to matter for them
Just because they don’t do full releases doesn’t mean it isn’t developed anymore. They switched to updating modules individually, with three updates made this month. Doesn’t sound very abandoned to me.