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.