I... I'm speechless. It's like you haven't read a single review of a PCI-E 4.0 SSD. So I guess here's a
link to Techspot's article on this exact matter.
Basically exactly like
@Lionvibez said, the price difference between a decent PCI-E 3.0 vs 4.0 isn't enough to recommend going for the slower drive and much cheaper NVME drives don't get close to the performance of either.
Why do you even bother with an NVME drive anyway? Why not stick to a cheaper Sata SSD if your argument is purely based on "you won't notice it".
On top of this, If you are a gamer, Game engines over the next few years will now start adapting to take advantage of SDD's since the lowest common denominator (Consoles) will now feature them by default. I think it's reasonable to assume a faster SSD will load a game faster over the coming years.