Yes, you can install a 1x PCIe card into a 16x or 4x PCIe slot

Graphic cards use the longest PCI x16 slots PCIe x1 interface has 36 pins arranged in pairs of 18 pins Out of 36 pins, only 6 pins are functional pins and the

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100 power hot slot ASUS - TUF Gaming NVIDIA GeForce RTX 4080 SUPER Overclock 16GB GDDR6X PCI Express Graphics Card - Black · XFX

100 super hot slot free A PCIe x16 slot by definition is a slot that will take a PCIe card with up to 164 pins and up to 89mm in connector length (Cf the Wikipedia