PCI Express 3.0 is relatively new, we currently at the stage where PCI-E 3.0 cards are not really even available on the market and PCI-E 2.0 is still the standard for the mainstream. The company PCI-SIG who make the industry standard connector that is the PCI Express ports have announced they are developing and progressing well with the PCI Express 4.0 interface. PCI-E 3.0 is double the bandwidth of PCI-E 2.0 and so in a logical fashion PCI-E 4.0 is double the bandwidth of PCI-E 3.0. Meaning a PCI-E 4.0 4X slot has the same bandwidth as a PCI-E 2.0 16X slot which is an impressive feat and should mean that bandwidth will no longer be an issue with regards to multi-GPU configurations like it is currently on the Gen2 slots. Although that said currently we very rarely saturate the bandwidth of the PCI-E lane itself it is more the PCI-E controller on the processors and motherboards that is the issue.
PCI-E 4.0 will produce 16GT/s and be physically the same as PCI-E 3.0 (and therefore PCI-E 2.0). There is also a big question “what do we need all this extra bandwidth for?” and currently we do not really have anything that requires that much bandwidth but in future it may be practical. All that said PCI-E 4.0 won’t arrive until 2014 at the earliest and the production costs of it will not differ at all to that of the current PCI-E 2.0 lanes. PCI-SIG also hint that they are working on ways to extract more power efficiency out of PCI-E lanes and meet the “power, cost and compatibility” goals of the future.
Source: Press Release