1 x pci express x16 slot running at x4 pciex4 - AIIMS

Physically yes, the slot connector for an x4 is generally smaller and PCIe is designed that smaller cards can occupy larger slots if needed

According to PCIE Expresscard , a x1 card can be inserted into the x4, x8, and x16 connectors 15 This is referred to as up

pcie slot cover PCIe cards can always operate in PCIe slots with the same or more lanes than the card For example, an x8 card can operate in a slot with x8, x16, or x32 lanes

pcie 4 gpu in pcie 3 slot Yes, you can plug into 16x slot, but off course it will run in 4x speed Some motherboard even let you plug 4x card into 1x