DuPont Co. today announced strong fourth-quarter and annual operating results, despite significant market and economic challenges. The Delaware-based parent of DuPont Pioneer reported earnings of 71 cents per share, compared with 59 cents per share in the year-ago period. The company also announced plans to buy back up to $4 billion of its stock using a dividend expected from the spinoff of its performance chemicals unit, Reuters reported. Operating earnings increased by 20 percent year-over-year in the quarter from a number of company actions, including strategic portfolio initiatives, continued productivity improvements related to its operational redesign, reductions in performance-based compensation and share repurchases, DuPont officials said in a release. For the full year, DuPont reported operating earnings of $4.01 per share compared with $3.88 per share in the prior year.