RTX 4090 into PCIe x8 slot

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)

It is used for GPUs mostly as well as SSD cards PCI Express x16 Slot – This is the largest and used for high-performance GPUs How Does PCIe Slot Work?

pcie slot function Risers enable GPUs to be oriented and placed in other locations apart from the actual PCIe slot With only one GPU, some tasks simply cannot be

pcie slot for gpu Most new high end graphics cards are normally dual slot now days, so that means the card chosen most likely cover two