For those of you who haven't yet tapped into Big Finish and feel daunted by the back catalogue, may I suggest some for starters?
Spare Parts - 5th, Nyssa, Cybermen
Creatures of Beauty - 5th, Nyssa
Loups-Garoux - 5th, Turlough
Phantasmagoria - 5th, Turlough
Jubilee - 6th, Daleks
The Holy Terror - 6th
Project: Twilight - 6th
Bloodtide - 6th, Silurians
The Genocide Machine - 7th, Ace, Daleks
Live 34 - 7th, Ace
The Harvest - 7th, Ace
The Fearmonger - 7th, Ace
The Magic Mousetrap - 7th, Ace
The Chimes of Midnight - 8th, Charley
Seasons of Fear - 8th, Charley
Neverland - 8th, Charley with Romana
What do we know so far about the 4th Doctor adventures? Six releases from Jan 2012 with Leela, seven later on with Romana, plus 2 'missing' stories made from unmade tv scripts (again with Leela). The Daleks are due to return. Now, 6+7+2=15 and Tom has stated he's doing 18. My guess is Elisabeth Sladen with be back at some point...