WASHINGTON — With mortgage rates sliding for a fifth straight week, the possibility of locking in a rate below 4 percent is tempting for consumers and could unleash a wave of refinancing. It may even convince some Americans to buy their first home.

Yet there are limits to how far the wave can extend. Millions of homeowners already refinanced in 2013, when the average 30-year mortgage rate stayed below 4 percent until mid-year. And the overall housing market remains hampered by tight mortgage credit, rising home prices and stagnating incomes.

This week the average rate on the 30-year loan fell to 3.92 percent, mortgage company Freddie Mac reported Thursday. The average for a 15-year mortgage, a popular choice for people who are refinancing, retreated to 3.08 percent from 3.18 percent.

That is sparking a boomlet of homeowners looking to refinance as long-term mortgage rates plummet. Homeowners eager for a bargain rate are firing off inquiries to lenders. Applications for “re-fi’s” jumped 23 percent in the week ended Oct. 17 from the week before — reaching their highest level since November 2013, figures compiled by the Mortgage Bankers Association show.

The average 30-year mortgage rate nationwide that week breached the 4-percent threshold and hit 3.97 percent from 4.12 percent the previous week. It was the lowest level since June 2013. Deepening concern over the health of the world economy compelled investors to flee stocks and move into bonds. That pushed up prices of Treasury notes and suppressed their yields, which often pull along mortgage rates.

But it remains to be seen whether the uptick in refinancing will turn into an extended boom.

About half of all homes with a mortgage have rates at about 4.3 percent or less, according to real estate data provider CoreLogic. It may not be worth it for those homeowners to refinance at current rates because refinancing carries its own costs and fees.

Nela Richardson, chief economist at real estate brokerage Redfin, estimates that only homeowners with rates higher than 4.25 percent would see a net benefit from refinancing.

Similarly, with many home borrowers already in the range of 3.5 percent to 4 percent on their mortgages, the spike in refinancing may not last very long, said Michael Fratantoni, the MBA’s chief economist.