At the 00:01:17 mark in the opening - as Japp is writing his "In Memoriam" letters - the top one is addressed to Miss Bulstrode, the headmistress of Meadowbank, seen in #11:2 - Cat Among the Pigeons (2008).
The episode title also works on 2 other meanings. It's the first of the final 4 episodes. Also, the big four could also be referencing the four-piece cast and character ensemble of Poirot, Japp, Hastings, and Lemon. All four had not appeared together since series 8 episode, Evil Under the Sun (2001). Hastings had appeared in one episode after that, Murder in Mesopotamia (2001), but not with Japp nor Lemon.
Jack Farthing (as Gerald Paynter), as of 2018, has now appeared alongside 2 different Poirots. David Suchet here in this rendition, and John Malkovich in The ABC Murders (2018).
Mark Gatiss also wrote the scripts for Cat Among the Pigeons (2008) and Hallowe'en Party (2010), and appeared on screen in Appointment with Death (2008) as Leonard Boynton.
Miss Lemon replaces mrs Pearson from the book