Wrath of the Iceni, by John Dorney, is the only Fourth Doctor story I can think of which is a pure historical - no sfnal elements at all apart from the Doctor and Leela, turning up in first-century Norfolk to encounter Boudica in the battle against the Romans. The story is a particularly good one for Leela, who becomes fascinated with the warrior queen (once it's been done, you realise that Boudica is the most obvious historical character for her to meet); and we get some twists on the "you can't rewrite history" theme. The first time I listened to it I was doing the family supermarket shoopping, and it seemed to me that the character development for Leela was too abrupt; listening to it again, while changing trains on a long journey through France (which I guess meant I was less distracted) it seemed to work much better. So a bit of a health warning that it depends on your frame of mind, but having read and listened to literally all the spinoff stories featuring Leela, I feel that this is one of the best ones for her in terms of character.
Energy of the Daleks was the first story Tom Baker recorded with Big Finish, but was released fourth in this sequence and also after the two Lost Stories. It has some good moments - notably the return of the Robomen - but some silly bits as well - at one point several characters teleport into someone's bed. Baker, Jameson, and particularly Nicholas Briggs as voice of the Daleks, author and director are clearly having tremendous fun, but I think it was wise to shift this one down the release order.
Alan Barnes has rarely disappointed me, and I'm glad to report that in his double story that ends this season, Trail of the White Worm/The Oseidon Adventure, he is on top form. Geoffrey Beevers returns as the Keeper of Traken Master, the idea being that he absorbed enough energy in The Deadly Assassin to become a bit less putrescent as the Doctor puts it. (There's nothing in The Keeper of Traken to contradict an earlier meeting between the Fourth Doctor and the Beevers Master.) In fact the standourt performance in Trail of the White Worm is Rachael Sterling, daughter of Diana Rigg who is to appear with her mother in a Mark Gatiss episode of the coming New Who season, playing a posh woman with more to her than meets the eye. The two stories are more separate than one normally gets in two-parters, though each still has both the Master (Beevers has good rapport with Baker, but isn't quite as evil as most Masters) and a wonderful demented colonel played by Michael Cochrane (who appeared twice in Old Who, as Charles Cranleigh in Black Orchid and Redvers Fenn-Cooper in Ghost Light). The Oseidon Adventure, not surprisingly given its title, is to a large extent a remake of The Android Invasion, with a lot of the same plot elements but doing it much better - particularly the confusion of identity of working out if you yourself may not unwittingly be your own android double.
I don't think any of these is really very penetrable for non-Who fans - perhaps the most accessible is Energy of the Daleks, which is also the least good. The others are all excellent news for those of us who are still in part in the Fourth Doctor era.