What is the Difference Between PCIe Gen 3 & Gen 4?

As mentioned above, a single PCIe slot can have x1, x2, x4, x8, or x16 lanes For example, PCIe x8 refers to a Gen 4 expansion cardslot

Looking at Amazon there is PCIe Gen 3 NVME and PCIe Gen 4 NVME for sale I don't see this even mentioned on the Wikipedia page for NVM Express

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