Wal-Mart boasts that its new $4 generic drug program is disrupting the market, attracting new customers to its stores and starting the nation on a road that will ultimately squeeze billions of dollars from prescription drug spending. “I was never a customer of Wal-Mart,” said Frank Ganci, 74, a retired independent contractor who lives in Ridgefield, N.J. He has no drug insurance, despite being eligible for it under Medicare, because he considers the monthly premiums too high. Mr. Ganci said he recently paid $12 for a month’s supply of three generic drugs at the Wal-Mart in Secaucus — atenolol for...