It is important to keep in mind that, at AM4 platform, only the PCI Express lanes controlled by the CPU (24 on total, 16 for the video card(s), four for an M.2 slot and four for the communication with the chipset) are PCI Express 3.0; the PCI Express lanes controlled by the chipset are use the 2.0 standard.
The ASRock Fatal1ty X370 Gaming K4 comes with two PCI Express 3.0 x16 slots and four PCI Express 2.0 x1 slots.
The PCI Express 3.0 x16 slots share the same 16 lanes, so they can work at x16/x0 (if the second one is unused) or x8/x8 configurations.
There are also two M.2 slots, the first one supporting both PCI Express 3.0 x4 and SATA-600 SSDs, and the second one supporting SATA-600 and PCI Express 2.0 x2 devices.
There is a third M.2 slot, supporting 2230 Wi-Fi cards.
The motherboard supports CrossFire and SLI with up to two video cards.
The two PCI Express x16 slots are covered by stainless steel shields, that work as a shield against interference, and also improving mechanical strength of each slot.
Figure 2: slots