I have built a UI application using Qt 5.7 and an independent widget using Qt 4.8. The widget uses QGIS API, which is incompatible with Qt5. Is there a way to run the widget from the Qt5 based application?
It cant, the Binary Compatibility of Qt5 means that a library build with Qt 4.8 will not run on it. The Api Compatibility was mostly kept, so if you could compile the QGIS on Qt5, then you could run your application on it.