Can you use an NVMe SSD on an older motherboard via a PCI

I want to know if my laptop has a GEN 3 NVME SLOT Or a G can i put a gen 4 m 2 in a gen 3 slot Testing and installing a Gen4 NVME SSD on a Gen3 Motherboard This includes can i put a gen 4 m2 in a gen 3 slot Two types of PCIe slot on an Asus H81M-K motherboard Various slots on a computer motherboard, from top to bottom: PCI Express x4; PCI Express x16; PCI Express

can i use pcie 40 in 50 slot The M2 Ultra chip provides 32 lanes of PCIe gen 4 to the system, with 8 lanes dedicated to the internal SSD The M2 Ultra chip connects to the PCIe slots