NEW YORK – Mortgage rates fell to a record low for the fourth time in five weeks. But low rates haven’t been enough to lift a struggling housing market.

The average rate for 30-year fixed loans last week was 4.56 percent, down from 4.57 percent a week earlier, mortgage company Freddie Mac said Thursday. That’s the lowest since Freddie Mac began tracking rates in 1971.

The last time home loan rates were lower was during the 1950s, when most mortgages lasted just 20 or 25 years.

The rate on the 15-year fixed loan dropped to 4.03 percent, down from 4.06 percent a week earlier and the lowest on records dating back to 1991.

Rates have fallen since the spring. Investors worried about the European debt crisis have shifted money into the safety of Treasury bonds. That has forced those yields down. Mortgage rates tend to track yields on Treasury debt.

However, low rates have yet to spark home sales and refinancing activity remains moderate.

Sales of previously occupied homes fell in June and are expected to keep sinking. The National Association of Realtors said Thursday that last month’s sales fell 5.1 percent to a seasonally adjusted annual rate of 5.37 million.

The housing market stalled after federal tax credits for homebuyers expired at the end of April. Home sales have dropped off, homebuilder confidence has waned and consumer sentiment is in the dumps.

It’s unlikely low mortgage rates will bolster housing. Rates have hovered near historic lows for more than a year, so many people have already taken advantage of them to buy or refinance a home. And many of those who haven’t wouldn’t qualify for a loan. They either owe more than their homes are worth, have shaky credit or have lost their jobs.

To calculate the national average, Freddie Mac collects mortgage rates on Monday through Wednesday of each week from lenders around the country. Rates often fluctuate significantly, even within a given day.

Rates on five-year adjustable-rate mortgages averaged 3.79 percent, down from 3.85 percent a week earlier. Rates on one-year adjustable-rate mortgages fell to an average of 3.70 percent from 3.74 percent.

The rates do not include add-on fees known as points. One point is equal to 1 percent of the total loan amount. The nationwide fee for loans in Freddie Mac’s survey averaged 0.7 a point for 30-year, 15-year and 1-year loans. The average fee for 5-year loans was 0.6 of a point.