Looking at the schematic, it doesn't appear to physically go anywhere at all, not even to a test point. I'm sure you could write code to configure it as an input or output, but how would you connect something to it?
Since PE4 is used as on-board I/O for other Minicore modules, it wasn't included in the PCIe pinout. To remain compatible with the other products, the RCM67xx modules don't make use of it either.
Is there something you can do with PE4 that you can't do with one of the other I/O pins on the PCIe connector?