From what I know my motherboard is PCI 4.0.yes you lose a little performance, plus you lose a little performance with the riser cable also since its not as good as putting a gpu straight into a mobo.
if you get a pci 4 riser cable you still need to have a mobo with pci 4 also
we are only talking a few % here though, nothing major and you probably will never notice it.
From what I know my motherboard is PCI 4.0.
So its really down to get the pci 3 for under £40 and lose a couple of % or pay an extra £30 for Gen 4 to gain a couple more but still wont be full.
Sounds like if I really want the vertical mount Gen 3 is better option.
This. If you ever need to reset your BIOS or update it etc you will need to connect the GPU straight to the motherboard to be able to configure the slot to 3.0 again, otherwise you risk getting no picture out.The gen 4 cable will give you that convenience of not having to play around with mono settings, so plug in and play.