Number Four is still at large
And Poirot knows it, or at least suspects.
Around the time we "learn" Number Four is Claud Darell, we also learn that Poirot has three other suspects in mind - one which disappeared after moving to Australia, one which disappeared after a nervous breakdown, and one which simply disappeared. We also learn that Poirot has been advertising for the whereabouts of all four of these men, and then the Peroxide Blonde, Miss Monro, shows up and conveniently tells our heroes how to identify our supposed villain, then promptly dies in a car accident. Poirot is immediately taken in, at least at first, and when next we see Number Four he's doing the exact thing the Miss Monro said he would do.
This is where the "Wild" part of "Wild Mass Guessing" comes in: Number Four is actually John St Maur, the one who simply disappeared. The real Number Four saw the ads himself, and planned accordingly. By sheerest luck, St Maur knew Darell (henceforth "the Stooge") and roped him into his plot. The problem was, the Stooge, like the real Number Four, was a character actor, who sinks himself into the parts he plays. So when the Stooge got the brilliant idea to dress up as a Chinese man and torment Hastings into giving up Poirot, he just went ahead and did it, without consulting the real Number Four.
It was at this point that St Maur realized he done screwed up. To cover his tracks, Four sent Miss Monro, an old mutual acquaintance, straight to Poirot, making sure she saw the ad for Darell, then told the Stooge about it, just a little too late, to see how he'd react.
From that moment forward, "Number Four" was actually the Stooge, thinking he was Number Four, right until the bitter end. And it was that bitter end that raised Poirot's suspicions - and caused him to follow up on those other three he so casually tossed aside the first time.
