Given the standard for PCI Express 4.0 is now over 6 months old since it was ratified, it’s reasonable to expect that the next batch of video cards will now support it.
Historically video cards have moved to the latest PCI Express standard. So we expect AMD and NVIDIA to carry on with this tradition.
Video cards do not need that much bandwidth to be able to deliver excellent performance. Most can do find even with PCi Express 3.0 and 2 lanes.
PCI Express 4.0 is double the speed of PCI Express 3.0. The rise of PCI Express SSD products are the biggest driver for faster speeds. Many M.2 SSD drives advertise speeds close to or above 2000 MB/s. A 10GbE networking card can be handled by a single lane PCIe 4.0 link.
M.2 SSD slots need 2 lanes and U.2 ports are designed for 4 lanes. This places a lot of demand on the CPU and chipset to deliver enough lanes to handle the bandwidth needed. The biggest problem with M.2 is that the amount of card space is limited as the technology was designed largely for mobile machines.
AMD is using PCI Express 3.0 with socket AM4 which is adequately fast for gaming. AMD will likely move to PCI Express 4.0 with the move to DDR5 which is expected to be standardized next year. AMD provides PCI Express 2.0 slots for the x1 and x4 slots on socket AM4 motherboards.
Intel is also using PCI Express 3.0 with their gaming motherboards. Intel may move to PCI Express 4.0 when they replace their existing X270 chipset as the 10nm line firms up.
PCI Express 4.0 will not help the shortage of lanes seen with both AMD and Intel consumer processors. This means the southbridge will be called on to handle more demanding peripherals. 16 lanes are needed for a video card slot or a pair of 8 lane slots for dual card machines. More lanes are needed for the LAN and southbridge which makes the case for 24 lanes from a CPU.
The crossbar design is not thermally demanding but it does take up a bit of real estate on a CPU. Video cards with NVIDIA Fermi and above and AMD GCN and above use a crossbar switch to be able to managed each segment as fast as possible. The disadvantage for crossbar switch is quadratic complexity as more lanes are added. Bidirectional is achieved using dual crossbar switches.
PCI EXPRESS 5.0
PCI Express 5.0 is not expected to be ready until the end of 2019 or beyond. Designs need to be tested and validated before being released to be sure the goals are met. PCI Express 5.0 is double the bandwidth of PCI Express 4.0 and quadruple the bandwidth of PCI Express 3.0.
Faster networking speeds are the main driver for faster lanes. 100 gigabit networks need more bandwidth to prevent bottlenecks. More recent is the development of 802.3bs-2017 400 gigabit networking. Faster 1 terabit speeds are expected in about 5 years as costs come down and better 400 gigabit hardware is developed,