An update on their site today had this to say:
"The story, which is written by Peter Anghelides and stars Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann and Nicholas Briggs, is released in December and will be available only to subscribers of the main Doctor Who range.
In a departure to previous years, there are no plans to subsequently make this story available for sale. The Four Doctors will only ever be available to Big Finish subscribers."
What more reason could you need for getting yourself over to their site and signing up? And just to make things extra easy, here is a nifty little link to transport you to their site faster than even a TARDIS could. Clicky Clicky