Can PCIE X4 card be installed in PCIE X16 slot?

Can PCIe x8 Fit Into x16 Slots? can you put a pcie x4 in a x16 slot I've never tested it, but allegedly it will split a single x16 slot into an x8 x4 and x4 in canyon slot The PEX4TO16R PCI Express x4 to x16 Slot Extension Adapter lets you install PCI Express cards into x4 PCIe slots in a 1U rack mount chassis

can i use pcie 30 in 20 slot It means, you'll be able to plug in any x16 add-in-card but that card will only ever be able to run at x8 PCIe bandwidth on that bottom