byte != bits. One byte = 8 bits
H5GC4H24AJR is a 4Gb (gigabit) memory chip. 4Gb means 512 MB (megabytes) per memory chip.
512 MB (megabytes) * 8 modules = Total of 4096 MB (megabytes)
If you looked up the part number GV-RX580Gaming-4GD, you would have seen that your bios has the correct gpu and memory clock speeds:
AMD Polaris 20, 1340 MHz, 2304 Cores, 144 TMUs, 32 ROPs, 4096 MB GDDR5, 1750 MHz, 256 bit
www.techpowerup.com
Like I said before. There is
nothing strange about your card.