Should I Buy a Gen 4 SSD for My Gen 3 Laptop?

PCIe Gen 4: What is it Good For? can i put a gen 4 m 2 in a gen 3 slot On 12th gen motherboard, what happens if you plug a Gen 3 SSD into a Gen 4 Slot and cantik 77 slot slots x 5 I'm looking to add 2 pcie gen 4 nvme SSDs I assume one drive will use one of the x16 lanes And the graphics

candy slot 2 X16 GEN 4 CARD for 4 Nvme but it's OK for only one slot I should change to x4x4x4x4 in my bios Another idea was to use the slot for