KDE Software Compilation 4 (KDE SC 4) was the only series of the so-called KDE Software Compilation (KDE SC), first released in January 2008. The final release was version 4.14.3 in November 2014. It was the follow-up to K Desktop Environment 3. Following KDE SC 4, the compilation was broken up into basic framework libraries, desktop environment, and applications, which are termed KDE Frameworks 5, KDE Plasma 5, and KDE Applications, respectively. Major releases (4.x) were released every six months, while minor bugfix releases (4.x.y) were released monthly.

The series included updates to several of the KDE Platform's core components, notably a port to Qt 4. It contained a new multimedia API called Phonon, a device integration framework called Solid, and a new style guide and default icon set called Oxygen. It also included a new, unified desktop and panel user interface called Plasma, which supported desktop widgets, replacing K Desktop Environment 3's separate components.

One of the overall goals of KDE Platform 4 was to make it easy for KDE applications to be portable to different operating systems. This was made possible by the port to Qt 4, which facilitated support for non-X11-based platforms, including Microsoft Windows and Mac OS X. Versions 4.0 to 4.3 of KDE Software Compilation were known simply as KDE 4 – the name change being a result of the KDE project's re-branding to reflect KDE's increased scope.

This is a short overview of major changes in KDE Software Compilation 4.

The port to the Qt 4 series was expected to enable KDE 4 to use less memory and be noticeably faster than KDE 3. The KDE libraries themselves have also been made more efficient. However, tests reveal that KDE 4.4 has the highest memory utilization on default Ubuntu installations when compared to GNOME 2.29, Xfce 4.6, and LXDE 0.5. Qt 4 is available under the LGPL for Mac OS X and Windows, which allows KDE 4 to run on those platforms. The ports to both platforms are in an early state. As of August 2010^[update], KDE Software compilation 4 on Mac OS X is considered beta, while on Windows it is not in the final state, so applications can be unsuitable for day to day use yet. Both ports are trying to use as little divergent code as possible to make the applications function almost identically on all platforms. During Summer of Code 2007 an icon cache was created to decrease application start-up times for use in KDE 4. Improvements were varied – Kfind, an application which used several hundred icons, started up in about a quarter of the time it took previously. Other applications and a full KDE session started up a little over a second faster.

Many applications in the Extragear and KOffice modules have received numerous improvements with the new features of KDE 4 and Qt 4. But since they follow their own release schedule, they were not all available at the time of the first KDE 4 release – these include Amarok, K3b, digiKam, KWord, and Krita.

The most noticeable changes for users are the new icons, theme and sounds provided by the Oxygen Project. These represent a break from previous KDE icons and graphics, which had a cartoonish look. Instead Oxygen icons opt for a more photorealistic style. The Oxygen Project builds on the freedesktop.org Icon Naming Specification and Icon Theme Specification, allowing consistency across applications. The Oxygen team uses community help for better visuals in KDE 4, with both alternate icon sets and the winners of a wallpaper contest held by the Oxygen project being included in KDE 4. There is also a new set of human interface guidelines for a more standardized layout.

Plasma provides the main desktop user interface and is a rewrite of several core KDE applications, like the desktop drawing and most notably the widget engine. Plasma allows a more customisable desktop and more versatile widgets.