The PCI SIG (PCI Special Interest Group) made assurances that it will complete its draft specification for PCI-Express 4.0 interface by second half 2014, but the interface will feature a bandwidth of up to 16 GT/s, double the bandwidth that PCI-E 3.0 offers.
It is being told so that the PCIe 4.0 will have backwards compatibility, just like how we have for PCIe 2.0 and 3.0 interface. The new specification will allow problems created by large data application by providing double the bandwidth. This will be implemented for servers, workstations, desktop systems (including gaming rigs), notebooks, tablets, embedded systems, etc. It also assured that it provides lower product cost through pin reduction.
The Rev 0.5 of the specification was presented in Q1 2014, but rev 0.9 will be presented by Q1 2015. Even if the Rev 0.9 specifications are finalized, its going to take a while for us to see native PCIe 4.0 motherboards and add-on cards, especially graphic cards and PCIe SSDs.