Focal Reducers for the NexStar 4
The rear port of the NexStar 4 accommodates the same accessories as the Meade ETX-90/105/125. In the Astrophotography chapter of my book, The NexStar User's Guide, I suggested a focal reducer from SAC Imaging (http://www.sac-imaging.com) - they have changed their offerings and do not sell the same product.
CNC Parts Supply, Inc now offers a focal reducer that provides an approximate focal ratio of f/7 - http://www.cncsupplyinc.com/etxstuff.htm. Look for the "1.25" CCD Reducer / 'Field Doubler' for SCT & Maksutov Telescopes", item number R125.