BOSTON ( TheStreet) -- Act fast if you want to lock-in today's record-low 3.5% mortgage rates. Market watchers expect sub-4% home loans to go the way of the dinosaur by summer. "I think this may be the final once-in-a-lifetime opportunity for people to pick up mortgages below 4%," says National Association of Realtors economist Lawrence Yun, who predicts rates will hit 4% during the summer and 4.5% by 2014's first half. Thirty-year fixed mortgage rates have averaged 8.7% since record-keeping began in 1971, but have all but collapsed since 2008's global financial crash. Average rates fell below 5% in 2009 for the first time on record, then dropped to less than 4% in November 2011 and have stayed there since. But now, Yun and other experts predict today's rock-bottom rates will slowly rise during 2013 as the U.S. economy continues to recover and inflation risks grow. "My forecast is that higher inflation will force banks to charge extra interest to compensate for the loss of purchasing power in the money that consumers borrow," Yun says. He attributes the increased potential inflation primarily to the Federal Reserve's easy-money policies of recent years. Yun also says apartment rental rates -- the largest component of the government's inflation-measuring Consumer Price Index -- are rising as the number of U.S. households expands. "Higher rents will push up CPI, and higher inflation will mean higher interest rates for consumers as lenders try to make a profit," he says. Mortgage Bankers Association economist Mike Fratantoni foresees rates rising even faster than Yun does, predicting 30-year mortgages will average 4% by late June and hit 4.6% before 2015. "We expect that mortgage rates will increase gradually as the economy improves," Fratantoni says. Greg McBride of market tracker Bankrate.com also foresees 4% average rates coming this year -- but doesn't think house-hunters or refinancers should panic. "My advice to consumers is that they can take some time if they need to, build up savings, pay down debts and improve their credit scores," he says. "Neither home prices nor mortgage rates are going to run away higher ." Yun adds that even the higher rates he and others expect will seem ultra-low in historic terms. After all, average mortgage rates topped 10% in the early 1990s and hit 18.45% in 1981. "If rates rise to 4.5%, I think many consumers will still see that as a good bargain," Yun says.