Back then there was just a single "KDE" release, instead of separate frameworks, desktop and applications that we have now. KDE 4.0 being released meant that the frameworks used to build applications were now stable and that initial ports of desktop and applications on top of those frameworks were now available. They made it clear that end users should wait with switching until the whole DE stabilizes.
That's just how their versioning scheme worked. The exact same thing happened with KDE 2.0 and KDE 3.0, but there was a different audience back then and 4.x took much longer to stabilize (as Qt3 -> Qt4 migration was much heaver that others, including Qt4 -> Qt5), so it became a bigger problem there.
It didn't happen with KDE 5.0 because there was no KDE 5.0 - now you have separate releases with separate versioning schemes of KDE Frameworks, Plasma Desktop and KDE Applications (and many apps were still based on KDELibs4 on initial release anyway and only gradually ported to KF5 afterwards).