I can see why this is not one of the more well loved or more remembered Poirot stories. Beyond the isue that people didnt love the dorky dime store early spy stuff Christie liked to play with.
The truth is that Big Four is more a collection of smaller adventures and short stories stitched together into an over arching plot and some people might find that offputting.
I think it is a really fun and experimental way for Christie to play with her characters
Honestly if I was adapting this into a screenplay I can see the way I'd want to do it. Make each of the smaller mysteries into their own long format mystery with connective tissue around the Four until the climax.
Build tension. Play up that Poirot is really the only one who sees the Big Four's machinations even as he solves the puzzles and saves lives.
Guess what I am saying is ai want a 10 part Poirot miniseries of the Big Four a novel few people like enough to justify such a thing.