NEW YORK (CNNMoney.com) -- U.S. mortgage applications rebounded last week from 4-year lows as mortgage rates slipped across the board, an industry trade group said Wednesday.

The Mortgage Bankers Association said its seasonally adjusted index of mortgage application activity for the week ended August 4 increased 4.9 percent to 553.3 from the previous week's 527.6.

Applications had slipped to their lowest level in more than four years during the previous week.

Borrowing costs on 30-year fixed-rate mortgages, excluding fees, averaged 6.45 percent, down 0.17 percentage point from the previous week.

The MBA's seasonally adjusted purchase mortgage index rose 3.4 percent to 388.9 from 376.2 the previous week.

The group's seasonally adjusted index of refinancing applications jumped 7.1 percent to 1518.1.

The refinance share of applications increased to 38 percent from 37 percent the previous week.

Fixed 15-year mortgage rates averaged 6.10 percent, down from 6.28 the previous week. Rates on one-year adjustable-rate mortgages (ARMs) decreased to 5.96 percent from 6.18 percent.

The ARM share of activity decreased to 27.6 percent of total applications, its lowest level since March of 2004, from 27.8 the previous week.

The MBA's survey covers about 50 percent of all U.S. retail residential mortgage loans. Respondents include mortgage bankers, commercial banks and thrifts.

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Slim pickings for real estate vultures

Mortgage rates slide for second straight week