Yes you kinda can with an FPGA, but you gonna generally be limited to how fast you can clock. The RPi4 clocks at like 1.5 Ghz with 4 cores, and if you want 4 fast cores that will eat up a lot area.
A pretty fast softcore is gonna be like 500Mhz, some really optimized designs might hit close to 700Mhz on better FPGAs. Although when working with softcores you can a lot times get away with lower clocks since anything that would require a lot of cycles if written in software can a lot times be made as a block on the FPGA that your softcore just manages. Assuming your FPGA has enough area. Freeing up the softcore to do other things.
A pretty fast softcore is gonna be like 500Mhz, some really optimized designs might hit close to 700Mhz on better FPGAs. Although when working with softcores you can a lot times get away with lower clocks since anything that would require a lot of cycles if written in software can a lot times be made as a block on the FPGA that your softcore just manages. Assuming your FPGA has enough area. Freeing up the softcore to do other things.