WASHINGTON — Average U.S. rates on fixed mortgages fell this week for a fourth straight week. The low rates could give a boost to the spring home-buying season, which has started slowly.

Mortgage buyer Freddie Mac said Thursday that the average rate for a 30-year loan declined to 4.14 percent from 4.20 percent last week. The average for the 15-year mortgage eased to 3.25 percent from 3.29 percent.

Warmer weather has yet to boost home-buying as it normally does.

Home construction surged in April to its highest pace in five months, the government reported last Friday, but nearly all the increase came from the volatile apartment sector.

And sales of existing U.S. homes rebounded slightly in April, but the pace of buying remained below last year’s level, according to data released Thursday by the National Association of Realtors.

Home sales and construction have faltered since fall, slowing economic growth. A harsh winter, higher buying costs and a limited supply of homes have discouraged many potential buyers.

Mortgage rates still are nearly a full percentage point above record lows reached about a year ago.