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

No problem whatsoever Also, if you look closely, the bottom x16 slot is actually a x4 slot in the outer shape of a x16 slot See how the pins

Yes, it works just fine, obviously it's only work at x4 speed I would check whether the x16 slot can handle the power load of the

can you put pcie 4x in 16x slot 1 PCIe x16 slot; 4 SATA3, 2 x ; Graphics Output Options: HDMI & Display ; 1 Rear USB Gen2x2 Type-C, 1 Rear USB Gen2 Type-A

can you use ddr4 in ddr5 slot I went out and grabbed a MSI B75A-G43 Gaming board as a replacement which has 2x PCIe x16 slots, one running at X16 and one