void MainWindow::button_clicked(Qstring a, Qstring b, Qstring c, Qstring d)

class MainWindow : public QMainWindow {
    Q_OBJECT
public:
...
    Q_INVOKABLE void buttonClicked( const QString& a, const QString& b, const QString& c, const QString& d );
....
};

void MainWindow::buttonClicked( const QString& a, const QString& b, const QString& c, const QString& d ) {
   ...do stuff
   update(); //example
}

QDeclarativeView* view = ...your view
view->rootContext()->setContextProperty( "_mainWindow", mainWindow );

_mainWindow.buttonClicked("foo", "bar", "c", "d")