All I've read about PCIe 4 and 5 has to do with speed increases, namely for the enterprise level for network interconnections. I suspect since PCIe is intended to be backwards compatible they wouldn't remove the 75w limit, especially since GPUs are the only thing that I can think of which suck down more than 75w.
I do remember reading some article a while back on TomsHardware about PCIe 4.0 allowing up to 400w per slot, but that was incorrect and was corrected. I believe that dual 8 pin (300w) GPUs will be considered good in the official PCIe 4.0 spec, and is probably where the confusion came from. I'm not sure how much I'd trust thin PCB traces and wires to be able to carry a kilowatt of power without shorting out.
I posted that some time ago when I saw something on the newswires. AMD is known to be offering PCI Express 4 with the upcoming next series of processors.
I am not sure yet what Intel is going to do as there has not been much leaking out of their speculators yet.
PCI Express 4 will not help graphics cards but motherboards will benefit the most.