The 2023 Mac Pro has 6 gen4 PCIe slots, but it has no MPX slots, which are special PCIe slots for the R4i and video cards. The MPX slots provide additional power for use by the Pegasus R4i and Mac Pro video cards. The 2023 Mac Pro has no MPX slots and don't support the use of a Pegasus R4i.
However the Pegasus J2i can be installed in the 2023 Mac Pro.
EDIT: It might be possible to use the Pegasus R4i with a TB to PCIe adapter. There are several on the market. This has not been tested and it may or may not work.
EDIT2: A Thunderbolt to PCIe adapter cannot be used as it cannot provide extra power to the Pegasus R4i as an MPX slot does.