Runway, already the best funded among a cluster of startups developing artificial intelligence software to generate videos for Hollywood and more amateur filmmakers, is trying to strengthen that lead with a new round of financing.The firm is in talks with investors to raise $450 million at about a $4 billion valuation, according to a person involved in the deal and a person who spoke with Runway executives. General Atlantic, a New York private equity and growth-stage investor, is in talks to lea...