SSD/ m.2

I have a Asus b850 mobo. And I have 3 M.2 slots th one that’s pcie 5 I put my GPU in but I just play games and nothing else . So my question is can I put a Gen3 nvme ssd in a Gen4 pcie ? Because all gen4 ssd are so expensive. And if you guys have any links please send it . For now I’m ok with 500gb storage . Thank you