Recently built a computer with a Ryzen 5 and a B450 board from MSI. The only 3rd-gen compatible one I could find in stock these days was mATX and lacking some of the newer features like WiFi, Bluetooth.
If the 4th-gen are compelling and we feel ready to upgrade to the Ryzen 9 I'll have a good excuse to get a nicer motherboard.
Although I do find a bit disappointing that they use the same socket for so long and the chips physically fit but don't actually work, that's kind of how technology goes sometimes.