The number of serious disciplinary actions taken against Massachusetts doctors fell by almost half over four years, placing the state's medical licensing agency in the bottom 10 nationally, according to a report released by a consumer group. The Public Citizen Health Research Group said the numbers suggest that Massachusetts has relaxed its policing of problem physicians. But a spokesman for the Massachusetts Board of Registration in Medicine said complaints to the board fell by more than a third from 2007 to 2009, and the number of malpractice payments made by Massachusetts doctors has gone down since 2001.