By Uday Sampath Kumar and Eric M. Johnson (Reuters) -Boeing Co said on Wednesday it incurred $4.5 billion charges in the fourth quarter on its sidelined 787 program, obscuring the U.S. planemaker’s long-awaited return to positive cash flow fueled by rebounding 737 MAX deliveries. The planemaker sank to a loss after two quarters of profits because of the charges. Shares in Boeing fell 3.6% as the array of charges and uncertainty over the 787 program dwarfed a surprise return to positive free cash flow and plans to increase production on the 737 and 777. The quarterly results underscore the chal…