The additional subscribers represented a 53 percent increase over new net subscribers in the same period a year ago, the Washington, D.C.-based, company said. XM's closest rival, Sirius Satellite Radio Inc., reported 3.77 million subscribers at the end of the first quarter.
XM's number of net subscribers increased by 18 percent over the first quarter.
XM has more than doubled its subscriber base since the second quarter of 2004, when the company had 2.1 million customers.
"The momentum behind XM's subscriber growth is undeniable," Hugh Panero, president and chief executive for XM, said in a statement.
Panero attributed the growth to popular programming, such as coverage of Major League Baseball and commercial-free music; support from carmakers that have included radios equipped to receive XM broadcasts in some models, and the company's line of XM2go portable, handheld radios.
XM and Sirius, which has also signed deals with automakers, have been spending heavily on new programming to attract subscribers. Among the new programs Sirius plans to launch this year include Martha Stewart Living Radio. In January, the company is scheduled to debut shock jock Howard Stern's talk show, and Nascar coverage is scheduled to start in 2007.
Neither company is profitable, but both have reported big increases in revenue, as well as subscribers. XM boosted sales more than twofold in the first quarter, while Sirius's revenues jumped 365 percent.