By TOM MURPHYFive months after ending development of its own obesity treatment, Pfizer is accelerating its push into the rapidly growing field with a nearly $5 billion acquisition. Metsera Inc. has no products on the market, but its pipeline includes four programs in clinical development and one in mid-stage testing. Pfizer said the deal will add expertise and potential oral and injectable treatments. Pfizer currently has no obesity treatments on the market but has some in clinical development. Pfizer said the boards of both New York-based companies have approved the deal, but Metsera shareholders still need to OK it.