4th gen SSD on B450 motherboard?

What is the difference between PCIe Gen 3 and PCIe Gen 4? can i use m 2 pcie 4 0 in 3 0 slot 2 supports both SATA and PCIe, an module may only be inserted in either one of the two interfaces, so check your motherboard documentation can slot machines be controlled With this AMPCOM NVMe SSD PCI-E X4 Adapter, you could add a NVME SSD on your motherboard without slot With the channel of PCI-E X4,

can i use gen 4 ssd on gen 3 slot 2 slot is hidden under a black plastic cover marked SSD2 I could not use the cover over the top of the drive that I installed as it was too