4 x 4 slot ✓ PCIe Gen 4x4 UD85

A PCIe x1 slot for example, has one lane PCIe x2 has two lanes, x4 has four, and so forth Unlike the older PCI standard, which can send data4 x 4 slot,Shop PCI-Express x4 Internal SSDs on Watch for amazing 4 Lanes Internal Solid State Drive WDS100T3X0E $$; $74

Your assumption is correct This would be a slot with only four PCIe lanes connected, but it can fit a 16-lane card This is pretty common, 4 x 4 slot,Sonnet 4x4 and McFiver PCIe Cards

4 x 4 slot,Allows one x4, x8, or x16 slot to be expanded with up to four x1, x4, or x16 PCI Express slots; LEDs for displaying downstream and upstream PCI Express link