I don't think so you will notice any huge performance improvement by PCI-4.0 or 5.0. There should be none to zero differences because GPUs don't even use all the bandwidths from PCIe 3.0 (16x).
Even using PCIe 8x makes no difference at all. It's hardware based. PCI-e is backwards compatible as well.
PCIe 4.0 will basically solve some key problems, not with bandwidth, but rather with how lanes are assigned. With GPUs needing 16 and NVMe drives needing 4, the 20 to 24 PCIe lanes consumer motherboards currently have are quickly saturated. PCIe 4.0 could double the bandwidth or halve the lanes required for each device, letting users choose between lots of current gen devices or a few next-gen ones.