The PowerColor RX-580 8GB versions have the GDDR5 VRAM clocked at 2000MHz.
I'm thinking the previous owner flashed / modified your RX-580 4GB vBIOS so matches the 8GB variants' speed.
The GDDR5 VRAM is able to handle those speeds, a it higher actually.
IMO, it's is stable, just leave it at 2000 MHz.
Looks like the GDDR5 memory chips PowerColor used on this cards can reach 2250+ MHz (via overclock), especially if equipped with the Samsung memory chips.