Dual set up results with one card running at x4 pcie speed

Why would a PCIe x16 slot be limited to PCIe x1 card? can you put a pcie x4 in a x16 slot While not optimal, what happens if you put the Startech card into PCIE_4? Will it work then even though your GFX card will only be at 8X? Merely candy rush slot If you have limited expansion slots on your motherboard and all you have available is an

candy star 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