Qt 4.8.7 [2021] -

Many industrial machines, medical devices, and automotive interfaces were built on Qt 4. Changing the framework in these environments requires massive re-certification and testing. For these systems, "if it isn't broken, don't fix it" is the golden rule, and Qt 4.8.7 is the most stable version they can run. 2. Massive Legacy Codebases

Even though 4.8.7 was the most secure Qt 4 version, it is no longer receiving official updates. New vulnerabilities discovered in the last decade remain unpatched. qt 4.8.7

Alternate: https://download.qt.io/archive/qt/4.8/4.8.7/ Many industrial machines