Tl;dr: You lose about 1% of performance going from PCIE 5.0 x16 to 4.0 x16.
x16 4.0
Remember what we said about PCI-Express 5.0 x16 availability being spotty on mid-range platforms? This makes it crucial for us to see how the RTX 5090 performs on PCI-Express 4.0 x16. Since LGA1700 processors don't put out a dedicated Gen 5 NVMe connection, Intel's motherboard partners resorted to wiring out M.2 Gen 5 NVMe slots by subtracting lanes from the Gen 5 x16 PEG interface, reducing it to PCI-Express 5.0 x8, which is exactly the same bandwidth as PCI-Express 4.0 x16—an invaluable data-point for the RTX 5090. We are happy to report that performance loss in this mode is well contained, and you lose about 1% performance, across all three resolutions. There are barely any outliers to report about from our set of game tests.