Among the features present in digiKam 4.0.0 is a new tool to organize whole tag hierarchies, a new maintenance tool to parse image quality and auto-tag items automatically using Pick Labels, more porting to Qt widgets that are more forward-compatible with Qt5, important improvements to the Import tool, and there's also multi-core CPU support for several tools that need a lot of computational power. Among the multi-core friendly tools are for sharpening, contrast, noise reduction, and visual effects.
The digiKam 4.0.0 release is now fully ported to the Qt4 model-view while the last of the Qt3 support classes have been dropped... The digiKam software has not yet been ported to Qt5. DigiKam developers intend for the Qt5 porting to go smoothly once the KDE Frameworks 5 API is stabilized and released, which is happening in the months ahead.
Those wishing to find out more from today's large digiKam 4.0 release can visit digiKam.org.