I think with FPGAs in general you'd have trouble getting to RPi4 frequencies (700MHz stock, with boosts on top of that), and I doubt you'd have enough resources

I think with FPGAs in general you'd have trouble getting to RPi4 frequencies (700MHz stock, with boosts on top of that), and I doubt you'd have enough resources (LUTs etc...) to implement all the features you'd find in a fully featured ARMv8 CPU (with 4 exception levels, a complex MMU and just an insane amount of configurable behavior in general). I mean, hell, the TRM (https://developer.arm.com/documentation/ddi0487/latest https://developer.arm.com/documentation/ddi0487/latest) for the CPU in a RPI4 is over 11k pages long. I don't think that's something you could reasonably expect to reimplement on your own. There'd be nothing stopping you from implementing a simpler/reduced RISC-V processor though, with only the ISA extensions you want (or none at all).