can you put a pcie x4 in a x16 slot

Up to 128GB Unbuffered ECC UDIMM, DDR4-3200MHz, in 4 DIMM slots; 1 PCIe 4.0 x8, 1 PCIe 4.0 x8 (in x16 slot), 1 PCIe 4.0 x4 (in x8 slot)1 PCIe 3.0 x2 (in x4 slot)

Will I be able to get all 3 PCIE cards to work, or can I assume that the 2nd PCIE 3.0 x8 can't fit in a PCIE 4.0 x4 (in a x16 size).

You can use PCIe cards of a lower lane width (e.g., x2, x4, or x8) in a higher lane width slot (e.g., x16) ... Do I Need an Adapter to Put a Shorter PCIe Card in a Longer Slot?