can i put a pcie 4.0 in a 3.0 slot: PCIe Gen 4 vs. Gen 3 Slots, Speeds - trentonsystems.com. PowerColor Readies Single-Slot, 4-way CrossFireX Ready HD 6850 ... . Everything You Need to Know About PCIe 4.0.
For example, a PCIe 3.0 device may not be able to fully utilize the bandwidth of a PCIe 4.0
slot, and a PCIe 5.0 device may not be compatible with a PCIe 3.0 slot at all.
If you buy a PCIe 4.0 expansion card but only have PCIe 3.0
slots on your motherboard. You can still use PCIe 4.0 expansion cards with PCIe 3.0 motherboards, thanks to PCIe 4.0’s backward compatibility.
Can I Use a PCIe 4.0 Graphics Card in a PCIe 3.0 Motherboard Slot? ... You can use PCIe cards of a lower lane width (e.g., x2, x4, or x8) in a higher lane width slot (e.g., x16).