After the release of the KDE Plasma Workspaces 4.9 I have read quite often in the Internet that users and also Media assume that the next release will be 5.0. This is not the case, the next release of the KDE Plasma Workspaces, Applications and Plattform will be 4.10 to be expected at the beginning of 2013 (release schedule has not yet been finalized by the release team).
I do not know why people assume that there would be a 5.0 release but I guess it is related to the work on Qt 5 and KDE Frameworks 5. Also some people seem to assume that after 4.9 the 5.0 has to follow due to second number being single digit, but a simple look at e.g. GNOME would show that numbers can increase as long as one likes.
At the current time there are no plans yet for a 5.0 release of the KDE Plasma Workspaces and at least I do not see any reason to increment the first number even after the release of Framworks 5. I expect that there will at least also be a 4.11 release before we start to think about version numbering for software based on Frameworks 5.