Roots: The original brief given to Anghelides was to create a DW version of A Christmas Carol. The Sixth Doctor paraphrases from Pericles Act II (iii) ("Time's the king of men - and Daleks, too") The character names "Lady Cowen" and "Whitmore" are references to two friends of the author, Laura Cowen and Tony Whitmore. Faraday's library contains Charles Lever's The Irish Dragoon (1840) as well as works by Trollope.
Intertextuality: The Dalek prime, a variant model of Dalek, for appeared in the Target and Virgin novelisations of The Chase and Evil of the Daleks and BBC Book War of the Daleks, all by John Peel.
Fluffs: Kalinda's accent goes provincial: "Yer wha-?"
Technobabble: The inversion of a light wave cone produces a Hyperbolic orthoginality. Light cone generators need wetware to operate them
Dialogue Triumphs: "I see you understand now, Doctor. This is my destiny. I've met you in your future lives, I've seen it, and I run to it willingly."
The Sixth Doctor's beautiful description of a (false) Sobravivante: "Let me tell you what I can see: the major sun of Sobravivanti is pulsing large and low in the atmosphere tonight. There is a mackerel sky; wisps of torn cloud bubbling gold and orange across the horizon. There's a park nearby - crisp yellow leaves are swirling beside the bandstand like dancing children. The paths are deserted, but their brown earth forks point away in the darkened evening like fingers directing visitors homeward [...] can you see beyond the silhouettes of the high buildings of the smoke-wreathed city towards the mountains in the east; how the furrows on their dark escarpments look like melted chocolate? And westward, the distant sea, where tall ships carry their sails."
Dialogue Disasters: "Your talk of hyperselves of the present and simultaneity was enough to convince me of your credentials, Doctor."
Double Entendres: "I insist you keep your grunts away from my lab"
"Stick your magic digit in there if you would, Professor.
Continuity: The Jariden and Daleks are described by the Doctor as old enemies, the Jaridens having won a war with the Daleks. Jerads are grey-skinned biomechanoids with fingerjacks, implying that their technology incorporates 'wetware' applications. Jariden technology extends to a Jariden-Dalek hybrid, enhancing individuals using reverse-engineered Dalek technology (possibly Ulrich's grandfather was the first?). The laboratory bulkheads are made of bonded polycarbide (see: Links) Jaridens have a concept of hell and cannot see unaided in darkness.
During the Battle of Bajorica the Daleks turn captive Jaridens into robomen. The Daleks of Ulrik's era cannot be reverse-engineered. Ulrik is a later 'upgrade' of the Jariden forces of the battle of Bajorica. Similarly, the Dalek Prime is a model not yet in existence in the Bajorican dalek history, so it too is not recognised. Ulrik knocks his past self unconscious (the Blinovitch Limitation effect?)
The Doctor detects a 'temporal echo' powered by highly-dangerous levels of chronon leakage from the top-secret temporal experiments taking place in the vault. A null-temporal environment hasn't sufficient chronons to function a time loop (but the leakage does)
A light cone generator is needed to create [instability?] but energy from the vaults creates a temporal whirlpool, hence another TARDIS is needed to counter the effects, essentially creating a time loop. When a light cone inverts everyone in the surrounding unstable area is sucked into the time loop the Doctor creates.
Large time loops require more than one TARDIS to create them. Chronon leakage can cause driftage in the TARDIS' coordinates. Removal of a TARDIS from a time loop also removes any temporal instabilities.
The Seventh Doctor makes a capacitor from the remains of a Special Weapons Dalek. When the four Doctors meet in the Eighth Doctor's TARDIS, it is said that only the Eighth Doctor will remember the meeting.
Links: The Three Doctors (the Doctor's joke about redecoration - "and if you have, I don't like it!") A Dalek utters the now-ubiquitous "my vision is impaired!" cry. Remembrance of the Daleks (bonded polycarbide, the Special Weapons Dalek) The Dalek Invasion of Earth (Robomen)
Untelevised Adventures: The Eighth Doctor is en route to the Petrachi Collection
Location: The Vault of Stellar Curios
The Bottom Line: "It all comes down to hyperbolic orthogonality"
As subscriber bonuses go... et cetera. No, in fact, The Four Doctors is by far the best multi-part subscriber bonus story you will get from Big Finish to date, ticking as many boxes as it does (multiple Doctors, A-list baddies, intrigue, romps), without scaring the horses by having the Time Lord/s share more than a couple of scenes (and only one between them all.) That last part may give pause to some, but at least it doesn't get in the way of some quite adequate storytelling. Plus, the Eighth Doctor's referring to himself as his own (i.e. the Fifth Doctor's) "superior' is a mischievous touch.