I think any differences between gen3 and gen4 are more likely to be felt loading large scenery files into ram while flying at speed or down low over PG. IMO as yet initial load times are more likely relevant to single core cpu performance unless you only have mechanical HDDs which really are too slow. When we get it Direct Storage should speed things up considerably.
Edit: I’ve just read the OP again, if I understand rightly he has MSFS installed on the PCIe3 which as this makes up by far the largest part of the loading time the only time he could possibly save is for cpu to read his register of content and compare versions to the sim’s latest, meaning no more than seconds. Going back a couple of years I remember swapping my NVMe drives and with an empty community and just the Toprudder 103 solo added the gen 3 was 20 seconds slower than the gen 4’s 105 seconds. Of course much has changed since then including the rest of my system.