This is far more than a 4MB RAM expansion. 

This is closer to going from the N64 to Dreamcast. 

It's perhaps the first true 8th gen machine.

It's pretty much equivalent to a New3DS except on for the PS4 so it's still 8th gen, just as the New3DS is 8th gen.

Still 8th gen, it would have to be at least some 3x more powerful than rumoured PS4K to be 9th gen.

8th gen. If we count all the Sega Genesis add-ons as part of 4th gen, and the N3DS as part of the 8th gen, there's no reason to not consider the PS4K a 8th gen console, specially if it mostly plays regular PS4 games (it will probably have some exclusives).

Obviously 8th gen, if latest rumours are to be believed.

The whole "generation 8" thing is stupid. No current manufacturer is on their 8 th generation of consoles.

This is Sony's 4th generation, not their 8th or 9th.

WTF...
Of course it will be 8th gen, it is just an upgrade and not a new console...

Does it matter at this point?

Don't think so. Every game should be playable on the old version.