The stated reason AMD have put fourth to not support ryzen 4000 series on B-450 and X-470 boards is the flash memory sizes on older boards, however there are x570 boards with 16MB flash chips while B450 boards with 32MB such as the max series with MSI.

This move is purely anti consumer and something we would not expect from AMD, it hurts their public image, and will hurt their sales in the long term.This will mean less people upgrading to ryzen 4000, as well as some people being put off buying AMD products again. 

Edit :For a more descriptive explanation see below. 

This is not my post. I found this very well articulated comment made by SvD KILLSWITCH on YouTube, and it echoes my own thoughts, so I wanted to share it here.

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Okay, here are my thoughts on AMD's current official stance on Zen 3 support. This is a long comment, but I've followed AM4 and Zen since its early inception so I've got a lot of thoughts.

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Full disclaimer: I'm an early 1st-gen Ryzen adopter. I purchased a Ryzen 7 1700 and an ASUS ROG CROSSHAIR VI HERO towards the end of March 2017. I'm not affiliated with any of the tech companies, I'm just a PC hardware enthusiast who has a few words for AMD.

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AMD's official reasoning for discontinuing support for 300-series and 400-series chipsets is a load of bollocks. The size of the BIOS chip is irrelevant - it's entirely within AMD's ability to support Zen 3 on boards with 16 megabyte (128 megabit) flash chips. All that AMD would have to do is provide code for each series of Zen processor and allow the motherboard manufacturers to create multiple versions of their BIOS files as an intermediate step in upgrading from an older AM4-compatible processor to a Zen 3-based one.

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For example, my CROSSHAIR VI HERO has a 16 megabyte flash chip, and currently supports the entire range of AM4-compatible processors, from the codename "Bristol Ridge" 7th generation A-series APUs released in 2016 (based on 28nm Excavator), all the way through to the codename "Matisse" 3rd generation Ryzen processors based on 7nm Zen 2. If AMD were to provide the necessary BIOS code for their range of Zen 3 processors and that increased the size of the BIOS beyond 16 megabyte, ASUS could provide two intermediary BIOS versions as part of the upgrade process. One version of the BIOS would support only Excavator-based, Zen 1-based, and Zen 3-based CPUs. The other version of the BIOS would support only Zen+-based, Zen 2-based, and Zen 3-based CPUs. Users upgrading to Zen 3 would select and flash the appropriate BIOS before swapping their old CPU out with a new Zen 3 CPU model.

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All future BIOS versions can then contain a subset of the code for the CPUs compatible with AM4. For space reasons, ASUS might decide to remove the code for the older A-series APUs (Bristol Ridge), Ryzen 1000 (Summit Ridge), and Ryzen 2000 (Pinnacle Ridge CPUs and Raven Ridge APUs). That would leave them only shipping BIOS files with support for Ryzen 3000 (Matisse CPUs and Picasso APUs) and Ryzen 4000 (Vermeer CPUs and Renoir APUs).

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I understand that currently, AMD has unified their AGESA microcode into one big "ComboPi" version that supports all currently available AM4 processors, but it's likely that with the launch of Zen 3, they will be required to do what they did when Zen 2 launched and fork their codebase while delivering initial support for Zen 3. In fact, that's what AMD will be required to do even if they cease supporting older boards, because they'll have to explicitly keep Zen 3 code separate from the rest so that older boards can't be updated. It wouldn't be a lot of engineering work to provide one version of code for each Zen series to facilitate the one-time BIOS upgrade process on older boards.

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On the earlier point of AMD's claim that BIOS size limitations are the main factor in ceasing support for older motherboards, it's worth mentioning that while vendors like MSI released updated versions of their motherboards with 32 megabyte flash chips, there are still brand new X570 boards that only have the smaller 16 megabyte chips (Gigabyte for example), and yet they're obviously receiving support for Zen 3 just fine. Additionally, despite AMD's datasheets specifying that X570 doesn't support Zen 1 processors, the fact that AMD's "ComboPI" AGESA even exists means that most (if not all) X570 boards do in fact support Zen 1 CPUs and APUs just fine. It's the same deal for A320 and Zen 2 - most A320 boards received BIOS updates that allow the use of even a Ryzen 9 3950X on a lowly A320 motherboard.

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The more pressing point I want to make though, is that if this was the plan all along and the 500-series was to be the break away from supporting older boards, then why has it taken this long for B550 to release? For months and months now since the launch of 3rd gen Ryzen, the only mid-range option available to buyers was B450. If you weren't buying a higher end Ryzen 7 or Ryzen 9 CPU and didn't need PCIe 4.0, the universal recommendation has been to buy a B450 motherboard and wait for Zen 3. AMD themselves have leaned heavily into the longevity of AM4 as a platform as a selling point, but pushing that point for the past few years is completely disingenuous if AMD aren't going to support the only chipsets that were available for purchase alongside 3rd gen Ryzen.

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I've recommended several 2nd and 3rd gen Ryzen systems to friends and colleagues, and one of the reasons I have has always been "better future support than on a comparable Intel system". AMD are going to be shooting themselves in the foot if all the people who I suggested buy a Ryzen 5 3600 and a MSI B450M MORTAR MAX are SOL for upcoming processor support given B550 has been complete vaporware up until this point. Why deal with the uncertainty of AMD processor upgrades when you could instead opt for the consistency of Intel's two-generation support lifetime? At least that way you know when you'll need a motherboard upgrade.

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At that point though, if AMD relents and decides to support B450 due to its availability on the market, surely they have to support the sister X470 chipset too, otherwise they'll be screwing over buyers of their high end hardware. And then, both the X470 and B450 chipsets are largely identical to X370 and B350, so why should they be left out? So we come back to the top of the discussion, and have to wonder why are AMD even ending support in the first place?

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It's ultimately just AMD trying to cut costs and wash their hands of AM4's long term compatibility promise. It's easy for them to say "it's the socket that's lasted a long time", and be technically correct, but the socket is nothing without the chipsets and AMD has done nothing to correct peoples way of thinking, nor to inform us that 300 and 400-series motherboards would not be supported by Zen 3 in any of their roadmaps.

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With the transition to DDR5 so close, all AMD have done is muddy the compatibility argument of AM4 as a platform and throw away much of the good will they've built up over the last few years. Not only are buyers of Ryzen from the last couple of years getting screwed out of future support, but future buyers are left in limbo too, because AMD hasn't committed to either AM4 nor a new socket for 2021. It's entirely possible that if AMD continues ahead with this planned compatibility cut, that buyers of B550 might get one generation of useful support before AMD transitions completely to AM5 for 2021 and 2022. On the other hand, even if Zen 3 turns out to be a big step forwards from Zen 2, good luck convincing existing Ryzen owners to upgrade when we all know that a new socket is coming. What of AM4's longevity then? It turns into the same argument I have against LGA1200 - With DDR5 so close, why lock yourself into only Comet Lake and Rocket Lake when we more or less know that Alder Lake is coming?

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If AMD doesn't wake up and smell the roses, my almost guaranteed planned upgrade to Zen 3 will instead turn into a "wait until 2021/2022 to see what both AMD and Intel have in store for Zen 4 and Alder Lake".