An easier way, select M4's head and export as a wavefront obj.  The next window will ask either single frame or multiple frame (default single frame), click ok.  The window after that is critical, it is the body hierarchy to select the parts you would like to export together.  Make certain that weld all parts is selected. 

The other method, make M4 completely invisible using the hierarchy window, make your selections, then export as wavefront obj.