During Intel’s developer Forum more of the PCI-Express 4.0 specification was detailed. The new specification will deliver a huge leap in host bus bandwidth and power for add-on cards. The latest draft of the specification has the bandwidth set at 16 GT/s per lane, that is double the current bandwidth (8 GT/s) of the PCI-Express 3.0 specification. The 16 GT/s per lane bandwidth translates into 1.97 GB/s for x1 devices, 7.87 GB/s for x4, 15.75 GB/s for x8, and 31.5 GB/s for x16 devices.
The power delivery of the slot for the specification will be quadrupled from PCI-Express 3.0 as well. PCI-Express 4.0 should be able to deliver 300W of power (compared to the 75W of PCI-Express 3.0). This could eliminate the need for additional power connectors on graphics cards that have a power draw of under 300W. But this change could happen gradually as graphics card makers will retain backwards compatibility with older PCI-Express slot and still have the power connectors on the card.
PCI-SIG has said they plan to finalize the PCI-Express 4.0 specification by the end of 2016.