SAN FRANCISCO — Sony BMG, the music company, announced Thursday that it would become the fourth and final major label to begin selling digital music on Amazon.com, offering its entire catalog in the MP3 format by the end of the month. Skip to next paragraph Related Bits: Sony Drives Another Nail in the D.R.M. Coffin Blogrunner: Reactions From Around the Web The move by Sony BMG, which represents artists like Bruce Springsteen, the Foo Fighters, Santana and Justin Timberlake, further positions Amazon’s digital music store as a significant rival to the market leader, the iTunes store from Apple. “This is such an exciting day for us and our customers,” said Bill Carr, vice president for digital music at Amazon. “All four major labels will be part of our service. It means our customers will really have access to all the biggest artists in the world.” Sony’s embrace of the MP3 format is also the latest blow to the technology known as digital rights management software, or D.R.M., which is intended to prevent consumers from making unauthorized copies of digital material. In an open letter to the music industry last February, Steven P. Jobs, Apple’s chief executive, said his company would welcome the end of software antipiracy measures and a world where music from any online music store could be played on Apple devices. Since then, one by one, major music industry figures like Edgar M. Bronfman Jr., Warner Music’s chairman, have supported the notion that D.R.M. was doing more harm than good in the evolving digital music market.