Lenders are charging U.S. car buyers the lowest interest rates in at least four years as fewer borrowers fall behind on their payments, supporting demand for new vehicles, according to Experian Automotive. The average interest rate for a new-vehicle loan fell to 4.52 percent in the fourth quarter of 2011 from a rate of 4.84 percent a year earlier. This is the lowest rate since Experian began tracking auto loan interest rates in 2008. Buyers whose credit is nonprime or worse accounted for 23 percent of the new vehicle financing market in the fourth quarter, up from less than 20 percent a year earlier, according to Experian.