Can PCI cards work in PCIe slots? The answer is no. PCIe and PCI are not compatible with each other due to their different configurations. In most cases, there are both PCI and PCIe slots on the motherboard, so please fit the card into its matching slot and do not misuse the two types.
Can you put a PCIe 4.0 card in a 3.0 slot?
PCIe 4.0 is backward compatible. It's also forward compatible. So, you can insert PCIe 4.0 expansion cards into PCIe 3.0 slots, but your data transfer rate will be limited by the slower speeds of PCIe 3.0.
Can I use any PCIe card?
It's worth mentioning that all PCIe card versions are backward compatible. That is to say, any version of the PCIe card and motherboard can work together in the mode of the lowest version.