The nation's 38.4 million Social Security recipients will get a 4 percent benefit increase in January, the second highest boost in 6 1/2 years. The Social Security Administration said Friday the increase amounts to $21 a month for the average retired worker, who gets $516 a month. It said the maximum monthly benefitfor a worker retiring in 1988 at age 65 will rise $61, from $838 to $899.