Sirius Satellite Radio announced yesterday that they crossed the 4 million subscriber mark. It took them a little over 3 months to grow from 2 million (September 15th, 2005) to 3 million (December 27th, 2005). It took less than 3 months for Sirius to go from 3 million to 4 million (March 20th, 2006). If they keep up this growth rate, Sirius would end the year with over 7 million subscribers. Sirius’ stock is up significantly over the last week on this news. XM reached 6 million on January 4th, so they will need to announce something about passing 7 million in the next week or so to keep pace with Sirius. I’ll be updating my XM vs Sirius total subscribers graph when I get the latest numbers from XM.