The Big Four is an odd novel for Christie. It is extremely episodic, and a quote right at the end of the book seems to place it before The Murder of Roger Ackroyd, even though it was published after:
No, I shall retire. Possibly I shall grow vegetable marrows!
I turned to Agatha Christie: An Autobiography for answers. Apparently, Christie was so depressed after her mother’s death that she was unable to write. Her brother-in-law, Campbell Christie, helped her convert 12 short stories (written before The Murder of Roger Ackroyd) into a novel. This satisfied the demands of her publisher, and brought in much needed funds.
Quite a few people on Goodreads complain about the espionage storyline. While unusual for a Poirot book, spy stories are well within Christie’s purview – see The Man in the Brown Suit, and the Tommy & Tuppence series.
While it’s not Christie’s best work, it’s still lots of fun and quite funny! If you’re reading the Poirot novels in publication order, I’d suggest reading this one before The Murder of Roger Ackroyd – it makes more sense that way.
You surprise me, Hastings. Do you not know that all celebrated detectives have brothers who would be even more celebrated than they are were it not for constitutional indolence?
A BIBLIOPHILE'S STYLE 
That last quote reminds me of something Sherlock would say to Watson about Mycroft. 😀
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I think that’s what Agatha was going for. 😆
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