XM Satellite Radio has reached 4 million suibscribers.

CEO Hugh Panero noted that it took 23 months from launch for XM to reach one million subscribers, but the move from three million to four million took less than five months.

XM says it remains committed to hitting its target of 5.5 million by the end of 2005. Meanwhile, rival Sirius has just over 1 million subscribers and predicts it will hit 2.7 million by year’s end.

Neither company is yet in profit, but listener growth has been very fast compared with the growth of free to air terrestrial digital radio, which is called HD Radio in the US.