The U.S. Federal Reserve today sliced its key federal funds rate to 4.75% from 5.25%, the first downward move in the rate in four years.

U.S. stocks added heavily to earlier gains, with the Dow Jones industrial average up about 189 points to 13,592 shortly after the Fed’s 2:15 pm ET announcement.

The rate, which is the interest banks charge one another, had remained unchanged since a series of increases ended in June 2006. Most analysts had today expected a cut of a quarter point, with fewer expecting a half a point.

The funds rate has not been cut since June 2003, when it reached a 1958 low of 1% following 13 straight reductions over more than two years.

Today’s move follows the Fed’s decision on Aug. 17 to unexpectedly lower its discount rate by half a point. The rate is the interest it charges for direct loans to banks.

The actions are sparked by weakness in the U.S. housing market, particularly chaos in the subprime mortgage sector.