J. C. Penney’s chief executive admitted on Wednesday that he had made “big mistakes” in his turnaround effort, as the retailer reported a startling fourth-quarter loss of $2.51 a share, compared with the 24-cent-a-share loss analysts had expected.

In the year since the chief, Ron Johnson, introduced his ambitious new strategy, the company has lost $4.28 billion in sales and its stock is down about 55 percent. In his quest to “be the favorite store for everyone,” Mr. Johnson said the retailer had gotten some areas wrong, including marketing and an assessment that customers wanted simple pricing without constant sales.

Penney’s quarterly sales reflected little customer enthusiasm for the new approach. Revenue in the quarter, including the crucial holiday shopping season, fell 28.4 percent to $3.8 billion. Its net loss amounted to $552 million compared with $87 million in the year-ago period.

Sales at stores open at least a year, a measure retailers use to gauge like-for-like demand, fell by 31.7 percent. And Internet sales, which have been increasing at a fast clip industrywide, fell by 34.4 percent.