AMD has confirmed that there is no PCIe 4.0 for old motherboards. In the run-up there were hints that X470 and B450 with Ryzen 3000-CPU can handle PCIe 4.0.
Ryzen 5 3600 and 3600X come with six cores and launch with their brothers. Also compatibility information of Ryzen 3000 on older motherboards is available.
Ryzen 3000 brings PCIe 4.0 to end-customer computers for the first time. Gigabyte updates now indicate that this also works with X470 and B450 motherboards.
The new Ryzen 3000 series will start soon. A new leak shows that Asus is planning 12 X570 mainboards for launch and is expanding the ROG Crosshair series.