4 x 4 slot ✓ PCIE 1 to 6 PCI Express X4 20Gb Riser Card Adapter PCI-E Slot 4X

4 x 4 slot This slot can be used as x16, x8x8, x4x4x4x4 using Bifurcation In Case of my Truenas install a 4 x NVME Board was used with the x4x4x4x44 x 4 slot PCIEX16E-Q4S is PCIe x16 edge connector to four PCIe x16 Slot Interconnect Cabling solution Supports PCI Express Gen 4 Bandwidth Power

4 x 4 slotPCIe boards can fit into slots designed for their lane configuration or higher Plugging a x4 PCIe into a x16 slot is acceptable

4 x 4 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,