You have a point, there is no comprehensive version numbering scheme that is universally accepted. I think you are also wrong as the issue is about the very well accepted beta, RC and final release scheme, not simply number sequences.
No number sequence, such as 8.10 or whatever comes to mind, is going to tell you that the release is a beta, RC or final release. However, outside developer circles, people are going to assume a final release, and that is precisely why one should label a version as beta or RC if confusion is to be avoided. Obviously, confusion was not avoided with 4.0, and again, not everyone keeps track of developer blogs and announcements.
Obviously, KDE 4.0 was barely a release candidate to end users, so one could at least have expected the RC label. Had it been "properly" named, I would find it hard to imagine distros like Kubuntu or Fedora shipping a first release candidate in the manner 4.0 was delivered. It would have been an embarrassment to them. Instead, KDE was embarrassed. Needlessly.