Interest charges for 30-year, flat-rate mortgages averaged 9.25 percent this week, down from 9.36 percent last week and their lowest in nearly four years, according to a national survey released Friday by the Federal Home Loan Mortgage Corp.

Charges for 30-year, fixed-rate mortgages were at their lowest since falling to 9.07 percent for the week ending March 27, 1987.On one-year, adjustable-rate mortgages, lenders were asking an average initial rate of 7.59 percent, down from 7.68 percent last week and the lowest since 7.56 percent for the week ending May 18, 1988.