The PCI-E extenders can run at PCIE 1.0 X1 with no hashrate drop. What makes you think that you will benefit from PCIE 4.0? 
In current technology and algorithm yes, it will not benefit from it, but what I am saying is that in the future if developers adapt and make improvments can this technology be beneficial?
It will be beneficial to the future generation of graphic cards, this new technology opens up a whole new world for graphic cards producers like Nvidia and Amd.We should hear an official statement from these companies in order to know more where the benefits will be.