4x nvme pci-e X16 cards on servers

Here is AMD's Crossfire compatibility chart to help you understand which cards will work together in crossfire and yes x16 and x4 will work fine

If you have a board that does not do CrossfireSLI, then sticking a card into another slot will have no bearing on the first You could also get an enthusiast

can you use pcie 40 in 50 slot You can put anything smaller than the slot in just fine It's the PCI-e bracket screw that really holds things into place I'd get the

candy pop slot That means if you purchase a PCIe Gen 4 NVMe SSD today, you can use it immediately in your current system even if it does not support PCIe Gen 4 If you upgrade