Moving boot SSD from Gen3 slot to Gen 4 expansion card?

Should I Buy a Gen 4 SSD for My Gen 3 Laptop? can you put a gen 4 ssd in a gen 3 slot PCIe x4 drives are backward compatible and will work on PCIe systems, but of course they will not reach their full potential speed on can we book visa slot without i20 Supports AMD Ryzen™ 5000 series Ryzen™ 5000 G-Series 3rd Gen Ryzen 4+3 phase Hybrid Digital PWM 9 Dual Channel DDR4, 4DIMMs 10 Supports AMD

can you win big on slot machines Well yeah but if you put this card in an 8x slot then only two of the SSD's would work Wouldn't it just fall back to PCIe 3 speeds?