iTunes has sold over four billion songs to over 50 million customers since opening in 2003.

“We launched iTunes less than five years ago, and it has now become the number one music retailer in the world,” said Eddy Cue, Apple’s iTunes vice president. “We are thrilled, and would like to thank all of our customers for helping us reach this incredible milestone.”

The ranking comes from market research data from NPD Group’s MusicWatch survey that tracks consumer purchases. iTunes topped Wal-Mart based on their January and February 2008 sales figures, Apple said in a statement.

This is yet another blow to the hard copy, “old way of doing things” music shopping experience. And yet, it is has shown that legal music downloading can and does succeed in the US.