PCIe 3 and PCIe 4 are two different types of PCIe slots that work in different ways. PCIe 3 slots are typically found in older computers, and are limited to a maximum speed of 10 gigabits per second.
PCIe 4 slots, on the other hand, are more commonly found in newer computers, and can support a maximum speed of 16 gigabits per second.
Despite their differences, both PCIe 3 and PCIe 4 slots can be used with the same types of cards. However, because PCIe 4 slots can support higher speeds, it is worth upgrading to a PCIe 4 card if you plan on using a high-speed graphics card or other device.