As per detailed EM photographs of the Renoir die shared by Fritzchens Fritz, the Ryzen 4000 “Renoir” APUs may just support up to 16 PCIe 3.0 lanes on the desktop platform. Earlier, many reports had claimed that like the mobile platform, the desktop Ryzen 4000 parts would also be limited to 8 PCIe 3.0 lanes. The die photographs show that there’s indeed a possibility that the desktop parts may include 16 lanes. Even the mobile parts have the required components, except they’re disabled:
Note that there are five PCIe x4 lanes on the right end of the die which totals to 20. Now, in the case of the notebook processors, only two of these are enabled, while the rest aren’t used. It shouldn’t be too hard for AMD to turn another pair of x4 lanes on for the desktop processors, especially if this is something that Team Red planned ahead for.