I don't know about software but just run a performance test on an PCIe 4 NVMe 2.2280 SSD.
I'm satisfied that a PCIe 4 device will only perform to its max on a PCIe5. Your probably going to see the same outcome from PCIe 3 vs PCIe 4. I'm just guessing, but I would assume that the 5 will have twice the bandwidth of the 4.
If it ain't broke; don't fix it!