PCIe x16 GPU card into a PCIe x8 slot?

On current Intel single socket systems a PCIe switch is required to get 4 full X16 slots for GPU's The alternative possibility for 4 GPU's on

if the slot is open like that, you should be good to go if it's not open, you could a) use another riser to adapt the x8 to x16, or b)

what does bar mean on a slot machine Slots P1-C2 and P1-C4 are x8 slots with busses direct from the Processor Modules and can be used to install high-performance PCIe adapters

what slot machines hit the most with 4 GPUs the server would require a minimum of 64 PCI-E lanes available on both, the server motherboard PCI-E slots and the CPUs For instance, a single CPU