WASHINGTON — Rates on 30-year mortgages dropped for a fourth straight week to the lowest level in nearly four years, raising hopes that low rates will help spur a rebound in the hard-hit housing industry.

Freddie Mac, the mortgage company, reported yesterday that 30-year, fixed-rate mortgages averaged 5.48 percent this week, down from 5.69 percent last week.

It was the fourth consecutive decline and the third straight week that rates have been below the 6 percent level.

The new rate marked the lowest point for 30-year mortgages since they averaged 5.40 percent the week of March 25, 2004.

Economists attributed the decline to further weak news on the economy combined with the biggest reduction of a key interest rate by the Federal Reserve in more than 20 years.

Other types of mortgages also showed declines this week.

Rates on 15-year mortgages, a popular choice for refinancing, dropped to 4.95 percent this week, down from 5.21 percent last week.

Rates on five-year adjustable-rate mortgages declined to 5.13 percent, compared to 5.40 percent last week, while rates on one-year ARMs fell to 4.99 percent, down from 5.26 percent last week.