Ram speeds are dependent on the cpu, not the motherboard. That B450, just like any other B450 is intended for 2nd release Ryzen cpus, the 2000 series. Those cpus are limited in ability to the speeds listed, at the time of that bios release.
They don't add 3rd release info to the motherboard listing as that would not be correct advertising, if you bought 4000MHz ram and stuck in a 2700x, you won't get that speed.
3200MHz (OC) means 2 things. You will have to overclock the ram (manually or xmp/docp) to get that speed and there exists the potential you may also have to overclock the cpu in order to get stable at that speed. Many Intel motherboards with a B/H rating, do not support the faster than default cpu setting because the bios...