Qt4

Qt4 is the fourth major release of the Qt cross‑platform application framework, released in 2005 as the successor to Qt3. Developed by Trolltech and later by Nokia, Digia, and The Qt Company, Qt4 represented a substantial modernization of the framework while maintaining cross‑platform compatibility. It introduced a modular architecture, an enhanced signals and slots mechanism, and a new object model that relied on the Qt Core and Qt GUI libraries, along with the meta‑object compiler (MOC) to enable features such as introspection and dynamic properties.

The Qt4 release expanded the available modules and tooling. It included core libraries such as QtCore, QtGui, QtNetwork, QtXml, and others, along with support for internationalization, a robust build system via qmake, and GUI design tools such as Qt Designer. The graphics subsystem gained the QGraphicsView framework, which provided a flexible 2D scene graph framework suitable for custom widgets and graphics-intensive interfaces. The platform coverage extended to Windows, macOS, Linux, and various embedded targets, reinforcing Qt’s role as a truly cross‑platform toolkit.

Qt Declarative, introduced during the Qt4 era, provided a declarative UI framework based on its own language for defining user interfaces, designed to work alongside C++ back‑ends. Qt4 was distributed under a dual license model, offering commercial licensing as well as GPL and LGPL licenses for open source use.

The Qt 4 family progressed through multiple maintenance releases, with Qt 4.8 serving as the final feature release in the series. Support for Qt 4 waned as the ecosystem shifted to Qt 5, which introduced a new rendering architecture and a more modern module set. Qt4 remains foundational in many legacy applications and embedded systems.