Like a full car odometer, the Dow will roll over from 9999 to 0000. They will automatically dump all stocks. That will trigger a massive sell-off. The market will go into free fall. Investors will be wiped out.
Nothing happens, other than investors grow richer.
Place your bets on Scenario No. 2- with some caveats. Like many financial firms nationwide, Columbia and Bidwell tweaked their systems last year when the Dow Jones average broke the 9,000 ceiling for the first time. Bidwell worked with its stock quote vendor to assure so-called D1OK compliance. Columbia modified its automated telephone transaction system for shareholdors, reprogramming it to accommodate the extra digit for the Dow index.