PCIe NVMe Gen3 vs Gen4

PCIe NVMe Gen3 vs Gen4 m 2 gen 4 in gen 3 slot MSI PRO H610M-S DDR4 Motherboard, Micro-ATX m2 slot On 12th gen motherboard, what happens if you plug a Gen 3 SSD into a Gen 4 Slot and

m2 slot MSI PRO H610M-S DDR4 Motherboard, Micro-ATX

m2 slot types slot supports PCIe Gen 4 x4 SSDs, while the right slot supports only PCIe Gen 3 x4 SSDs or SATA SSDs 0 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