USB 4.0 is not on any sort of near-term horizon. The USB 3.0 Promoter Group recently announced a specification update to add a 10Gbps SuperSpeed USB capability to the USB 3.0 Specification. According to
this document, the 1.0 draft of this update will be available in the Q3 2013 timeframe, which is actually well ahead of what was initially announced. The other good news is that in addition to the nominal bitrate doubling, a more efficient encoding scheme will be used. A lot may happen between now and 2015, but it would be a miracle if Intel releases an updated xHCI spec and a chipset that integrates 10 Gbit/s SuperSpeed USB by then. This makes it highly unlikely that we'll see a Mac with this capability by 2015.
For historical perspective:
USB 1.1 spec released Q3 1998, integrated by Intel Q2 1999
USB 2.0 spec released Q2 2000, integrated by Intel Q2 2002
USB 3.0 spec released Q4 2008, xHCI 1.0 released Q2 2010, integrated by Intel Q2 2012
USB 3.0 update released Q3 2013, integrated by Intel...
Granted, the xHCI was designed to be eXtensible, so this update may take less time to roll out, but all the stars would have to align for Apple to ship a product including it just two years from now. However, in the first half of 2014, Intel will be ramping production of Falcon Ridge Thunderbolt controllers, which will provide a full 20 Gbit/s, full-duplex link and DisplayPort 1.2 with HBR2 and MST capabilities. USB 3.0 crushes Thunderbolt on price/performance for most consumer applications, however, Thunderbolt can do things that USB simply cannot. As long as USB is not a true digital display interface, and DisplayPort is still relevant, Apple will most likely include a DP port, and it might as well also be a Thunderbolt port.
You should know by now, Apple strives to cater to the 1%.