I'd avoid the first load of PCI-4.0 drives, as they offer very little over the performance of a high-end 3.0 drive and are not pushing the 4.0 (4x) PCI-E slot at all. They like to advertise 5GB/s, but even sequential only it will be more like 4.2GB/s, which is good, but you can get 3.0 drives that push 3600 MB/s read/3300 MB's write.
4.0 drives will start to become interesting once Samsung introduce their new controller later in the year, with higher density NAND chips at 128-layers, this will push drive beyond 6.5GB/s, and towards 7.5GB/s by the middle of next year.