Ryzen 4000 Zen 3 Desktop CPU Compatibility Only With X570 & B550, not 400 or lower

It's been a bit of an uncertain fact, however, AMD now has confirmed that the next generation of Ryzen processors will not be compatible with Series 300 and 400 chipsets, it, however, will remain compatible with B550 and X570.

The news is somewhat unexpected but was anticipated as most people expected Ryzen 4000 to be drop-in compatible on AMD Socket AM4 motherboards. Those who bought an X370 or B350 motherboard with the first generation Ryzen could later (through a bios update) add support for processors from the Ryzen 2000 and 3000 series. This was the case even on some A320 boards.

Earlier and cheaper motherboards use smaller bios (16MB) chips, that's less storage space than needed. As a result, it became increasingly difficult for motherboard manufacturers to continue to add support for newer processors; it simply no longer fits on the bios chips.  AMD will need 32MB BIOS ROM chips. AMD certainly gave series 300 and 400 a long lifespan as even most people did not expect to see ZEN2 supported on Series 400. In short, for Ryzen 4000 aka ZEN3 processors that are released in the future, require at least a B550 or X570 motherboard, mandatory.  With B550 and X570, you thus are guaranteed to get support for Zen 3 processors when they get released. Much like before, support will be added through firmware updates that motherboard manufacturers will make available.

Small interesting fact though, MSI did release some Series 400 mobos labeled "MAX" which have been specifically fitted with 32MB BIOS ROM chip. It will be interesting to see if such a motherboard will get Ryzen 4000 support.

AMD:

Q: Will the “Zen 3” architecture be compatible with AMD Socket AM4?
A: Yes! AMD officially plans to support next-gen AMD Ryzendesktop processors, with the “Zen 3” architecture, on AMD X570 and B550 motherboards. This will require a BIOS update. Specific details about this update will come at a later time, but we’re committed to keeping you up-to-date. We’ve also updated our official chipset/CPU support matrix to take future “Zen 3” processors into account (below).

AMD has no plans to introduce “Zen 3” architecture support for older chipsets. While we wish could enable full support for every processor on every chipset, the flash memory chips that store BIOS settings and support have capacity limitations. Given these limitations, and the unprecedented longevity of the AM4 socket, there will inevitably be a time and place where a transition to free up space is necessary—the AMD 500 Series chipsets are that time.