4) Terrible Lizards, by Jonathan Green
An attempt at homage to Conan Doyle's Lost World - the Doctor, Amy and Rory find themselves on a ship carrying explorers to seek the Fountain of Youth in Florida in 1880 and get stuck in a time rift with dinosaurs. It doesn't work terribly well, the plot being reminiscent of one of the more nonsensical TV episodes, Amy screams and bursts into tears at any minor setback, and the one black character is the first to be eaten by the Tyrannosaurus Rex.
5) Horror of the Space Snakes, by Gary Russell
This is much more like it. The Doctor, travelling alone, arrives on a Moonbase (explicitly the same one visited by his second incarnation) which is suffering unexplained pressure drops and disappearances. There's also a crowd of teenage visitors who have won a ticket to the Moon from their favourite TV show. There are, of course, space snakes, and lots of continuity references (and slight reimaginings) to satisfy long-term fans, and a fairly inventive ending which will satisfy most readers.
On the whole these 2-in-1 books have been about average but this is an unusual case of a weak story published along with a rather strong one.