Plasma Active 4 is said to be faster, smoother, and more usable than earlier Plasma Active releases. Plasma Active 4 also features improvements to the file manager, there is now integration support with the Plasma Media Center, synchronization support with the ownCloud personal cloud storage software, a new developer mode, and support for the Mer Open Build Service.
More details on Plasma Active 4 can be found via news.kde.org. "Plasma Active 4 is a stabilization and performance release, the result of developers focusing on fit and finish throughout. This release is intended to complete the evolution of Plasma Active to a polished product relative to its proof of concept first release. The emphasis is now on providing a solid foundation for third party applications, additional content and device adaptations. Plasma Active 4 is well-suited to customization for specific needs, use cases and markets. This release sets the stage for adopting newer technologies such as Qt5 and Wayland in future releases."