By DAVID KOENIG (AP Business Writer) The United Auto Workers and Detroit’s Big Three carmakers resumed talks aimed at ending a strike now in its fourth day, and under the threat that the walkout could soon spread. Stellantis said it resumed negotiations with the union Monday and described the talks as “constructive.” A spokesman for General Motors said representatives of the company and the United Auto Workers also were continuing to negotiate. However, UAW President Shawn Fain said on NPR, “We have a long way to go,” and if the companies don’t respond to the union’s demands, “then we will escalate action.” So far the strike is limited to about 13,000 workers at three factories — one each at GM, Ford and Stellantis, the successor to Fiat Chrysler. However, the union’s strategy hinges on its ability to escalate the strike quickly, and the carmakers are warning of potential layoffs as the limited strike reduces the amount of material needed at plants that remain open. GM said Monday that 2,000 UAW-represented workers at an assembly plant in Kansas City are “expected to be idled as soon as early this week” because of a shortage of supplies from a GM plant near St.