https://wccftech.com/amd-ryzen-4000-zen-3-vermeer-desktop-cpus-compatible-existing-am4-motherboards/

AMD's Zen 3 based Ryzen 4000 'Vermeer' Desktop CPU lineup would be the biggest and hottest launch of 2020. While the launch is planned for later this year, motherboard manufacturers, OEMs, and generally everyone is excited for the upcoming family which would replace the highly successful Ryzen 3000 series. Now, creators of the 16 core, 32 thread APEX 15 laptop, XMG, have confirmed that AMD's Ryzen 4000 series processors will be compatible with existing AM4 motherboard such as the ones based on the B450 chipset which leads to increased longevity of the AM4 platform.

According to the post by the XMG employee on a Reddit Thread (via TechPowerUP), it looks like several board makers will expand compatibility of their existing AM4 motherboards to support AMD's next-generation Ryzen 4000 'Vermeer' Desktop CPUs and also Ryzen 4000 'Renoir' APUs. This was confirmed for the XMG APEX 15 in its specification list as can be seen below: