When senpai Tom Baker finally realised that Dementia In Time was a horrible way to end his career of playing the Fourth Doctor, he decided to star in a trilogy of AudioGo plays with Mike Yates known as Horniest Nest. He enjoyed returning to the role so much that he suddenly felt extremely guilty for rejecting Nicholas Briggs' decade-long begging for him to resume the character in the Big Finish's audios. So, from encouragement from his fellow companion actors, he decided to join the cast list for
the paycheck the sheer joy of it, pleasing all fans everywhere with his almighty return.
Seasons 1 & 3 (and soon Season 7 as well) feature the Fourth Doctor and Leela as they were in the period between The Talons of Wang-Chiang and The Invisible Enemy, with the very first story of Season 1, Destination: Nerva, taking place immediately after the events of Talons. Season 4 throws K9 into the mix, placing these Four and Leela adventures after the events of The Invisible Enemy.
Season 2 pairs up Four with the Unsexy Romana and K9, set after the events of The Key to Time-Wasting but before the regeneration into Sexy Romana. There undoubtedly would've been more stories set in this time period, but Mary Tamm died before any of Season 2 was even officially released. Le sigh.
Season 5 reunites the long separated Tom Baker and Lalla Ward as Four and Sexy Romana to do a bunch of stories with K9 set during the Douglas Adams period. Rumours persist that Tom and Lalla record their lines at separate locations to avoid encountering each other, however these rumours are made up by autistic obsessive fans and should be ignored. Season 6 features the same characters but set during the JNT era.
Season 8 is set for release in 2019 and will feature Four pared up with a new companion, Laan Kelso.
The original plan was for Tom to do his first series with Sarah Jane Smith, as evident from the premise of Destination: Nerva (considering the fact that it's set on... well, Nerva. Why else would they have planned a story there other than to have Four and Sarah geeking out about how they've been there before?). Unfortunately Elisabeth Sladen died before any stories could be recorded and the season was rewritten to include Leela instead.
It's a good thing that they're so high quality; they are the only canon Fourth Doctor Adventures after all. Well, except Pescatons, of course.