4 x 4 slot ✓ x4 slot

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,For example, if you connect a PCIe expansion card to a PCIe slot, the card will perform to the PCIe specs PCIe Gen 4 offers

The Sonnet 8x4 Silent Gen4 PCIe Card enables you to mount up to eight NVMe SSDs into a single x16 PCIe card slot and combine their capabilities for ultra- 4 x 4 slot,4 Expansion Cards: Some expansion cards, such as those for additional USB ports or specialized processing tasks, may also use the X4 interface

4 x 4 slot, The reinforced steel slot is capable of supporting PCI Express standard - 4 x DDR4 DIMM Slots - 11th Gen Intel® Core™ Processors