For whatever reasons the "4" was affixed to the KDE components, it was that most major distros use it. The "4" has been dropped by these major distros as well. It was not just something we decided to do ourselves but patterned over what others have already been doing. It would also make searching at rpm.phone uniform and easier.
muungwana is right. It doesn't hurt to keep it and could be useful in the transition to KDE5. And though often the suffixes are hidden, we still have 4.9.4 to think of how we can facilitate the transition.
Peace and much respect,