Can I be 100% certain that a PCIe x4 card will fit in a PCIe x16 slot?

x1, x4 pci Cards Fit Into x16 pci-e Slot pcie x4 in x16 slot Physically yes, the slot connector for an x4 is generally smaller and PCIe is designed that smaller cards can occupy larger slots if needed pcie m2 slot Pcie x16 slot into multiple x4 slots · PCIE Bifurcation · IBM 69Y5321 M4 PCIe Gen-III Riser Card 2 · Cablem ATIC

pcie training error slot 1 Pcie 4x slot vs pcie 16x slot Will it impact gpu performance when inserted in low