Every iteration of the pci-e standards has doubled the potential bandwidth. That's all it's done. Any gains in IOPS or TRW or speeds of drives or gpu memory or clocks have come from the manufacturer, the abilities of the silicon, the upgrades in the controller technology etc.
But none of that speed affects any mouse, any keyboard which barely make use of the USB2.0 bandwidth, never mind anything bigger like pci-e. It's why Intel skipped pci-e 4.0 entirely on mainstream boards, there was no real gains to be had by investing in it with pci-e 5.0 on the horizon.
There's realistically only a few circumstances where 4 is honestly better than 3, and it's not in gaming or general pc usage. If you can't saturate the bandwidth, or at least use far more than 3 can provide, it's just potential, not actual.