In all the winnowing of billionaires revealed this week - the credit crunch has wiped $2tn (£1.4tn) off their combined worth, and 30% are no longer billionaires at all - one thing stands out: retail is a good place to be. Of the top 15 billionaires, nine made their money the old-fashioned way, by selling us clothes, food and furniture. And of those nine, four belong to the same family.

Jim, Alice, and Rob Walton are the children of Sam Walton, who founded Wal-Mart; Christy Walton is his daughter-in-law. Alice, Christy, and Rob are each worth $17.6bn, while Jim is worth $17.8bn; together, the clan are nearly as rich as Warren Buffett and Bill Gates (the top two on the list) combined.

Sam Walton began his conquest of the world in 1945, with a loan of $20,000 from his father-in-law and a small variety store in Newport, Arkansas, where he established the practices that define present-day Wal-Mart: he kept prices as low as possible, stocked a wide range of goods, and stayed open longer than anyone else. His margins were small, but he sold large quantities, which meant he could bargain for even lower prices from wholesalers - policies that still drive smaller local stores out of business.

Even in his later years, when he was worth $24bn, he was famously frugal, opting for $5 haircuts (no tip), and cheap food at his local Wal-Mart. He drove an old pick-up and often borrowed money from his employees. And he was ruthless. "Some people try to turn it into this 'Save the Small-Town Merchants' deal, like they were whales or something that have a right to be protected," he wrote in his autobiography. But he was having none of it. "What happened was as inevitable as the replacement of the buggy by the car." When he died, in 1992, the state got almost nothing in taxes, because he had divided his wealth between his wife Helen, who died in 2007, and his children.

Wal-Mart now has 4,100 outlets in the United States and a further 3,100 in 13 countries around the world, including Asda in Britain. Sales were over $374bn last year (only slightly lower than the GDP of Denmark). Wal-Mart employs more than 2 million people worldwide, meaning it has twice as many men and women in uniform than the US army. Because they have a 43% stake in Wal-Mart, the four Waltons have the potential for huge influence, in America and the rest of the world - and yet, because they generally refuse interviews, not much is known about them. A reporter for Fortune, strolling round Bentonville, Arkansas, was hard put to even find the offices from which their fortunes are run.

Except for an unsuccessful stint as a buyer, Alice, the youngest, has never had anything to do with the day-to-day business of Wal-Mart. Instead, she became a broker (at 25, she was briefly suspended for making "unsuitable" option trades). She founded her own investment company, Llama, and lobbied to have an airport in Bentonville. In 1989 she struck and killed a 50-year-old woman who stepped in front of her Porsche (she was never charged).

Nine years later she was arrested for drink-driving. "You know who I am, don't you?" she demanded of her arresting officer. She fought her conviction in court, but lost, eventually paying a fine of $925. She is currently ruffling feathers in the art world by buying up masterpieces for a museum she wants to establish in 2010 - in Bentonville.

For a long time John also avoided the family business. He served as a green beret and medic in Vietnam, Laos and Cambodia. On his return he ran a crop-dusting business (largely, it seems, so he could continue flying), built trimarans and motorcycles, learned aerobatics, skydiving and scuba, and campaigned for his version of school reform: charter schools, vouchers, ways to give families a choice outside the public school system. He joined Wal-Mart's board in 1990, and died in 2005 when his homemade plane crashed (Christy is his widow).

The Fortune reporter found that Rob Walton, company chairman (the CEO is a non-Walton, Mike Duke), worked in a small windowless room, 10ft by 10ft square - then again, he didn't seem to be there that often. He races vintage sports cars, and hunts geese in Canada and pheasant in South Dakota. Jim is a different story - a reputed workaholic, he didn't join the Wal-Mart board until 2005. Instead he ran Walton Enterprises, and a bank, Arvest, that he bought in the 1960s and now has over 200 branches.

He is a local newspaper and internet publisher too - in fact, anyone who lives in Bentonville probably shops in Wal-mart for food, clothes, furniture and electronics, banks at Arvest, and, until recently, read a Walton-owned paper. They can drive down Walton Boulevard to watch sport at the Walton Arena. They can wander around the Walton Arts Centre, or go to the Wal-Mart Museum, where old Sam's office and pick-up are preserved exactly as they were the day he died. They can study at the Sam Walton business school, or fly from the Alice L Walton terminal of the airport.

The concern is, of course, that this approach has been extended to the rest of the world. Wal-Mart has made a speciality of moving into depressed areas where every saving a customer can make is welcome. This undercuts local businesses, and soon, because it sells everything from food to rifles, clothes to furniture, it's the only game in town. Although they are called "associates", employees work for little in the way of pay or benefits, and are discouraged from joining unions. The credit crunch means millions more people are dependent on finding the cheapest goods they can. It might have been tailor-made for the Waltons.