Modify your meshgeneratorwidget.py to record grid points

click4points.py

def init(self)
    self._ui.sceneviewer_widget.grid = []

def keyReleaseEvent(self, event):
        if self._marker_mode_active:
            self._marker_mode_active = False
            self._ui.sceneviewer_widget._model = self._plane_model
            self._ui.sceneviewer_widget._calculatePointOnPlane = None
            self._ui.sceneviewer_widget.mousePressEvent = self._original_mousePressEvent
            if  len(self._ui.sceneviewer_widget.grid) is 4:  
                self._grid4 = self._ui.sceneviewer_widget.grid
                
def mousePressEvent(self, event):
    if self._active_button != QtCore.Qt.NoButton:
        return

    if (event.modifiers() & QtCore.Qt.CTRL) and event.button() == QtCore.Qt.LeftButton:
        point_on_plane = self._calculatePointOnPlane(event.x(), event.y())
        print('Location of click (x,y): (' + str(event.x()) + ', ' + str(event.y()) +')')
        
        # return sceneviewers 'mouspressevent' function to its version for navigation
        self._model = self.plane_model_temp
        self._calculatePointOnPlane = None
        self.mousePressEvent = self.original_mousePressEvent

        self.grid.append(point_on_plane)
        if len(self.grid) > 4:
            self.grid = []