I suspect that USB 3.0 is likely to be the last iteration of that brilliant standard. There just doesn't seem to be any way to cram more bandwidth into the cables and sockets while maintaining backwards compatibility, and even version 3.0 was a bit of a stretch. We may still get USB 4.0, but it will probably have to break the backwards compatibility of the standard. Whenever that happens, whether it's version 4.0 or later, it should be given a different name to avoid confusion.
Mokalus of Borg
PS - It will feel like a sad, strange day when USB is obsolete.
PPS - It's going to happen, though. Someday.