Agatha Christie's Poirot: Four and twenty blackbirds

A not surprising but still interesting little thing, where there actually is no case officially, that is until Poirot solves it thus proving that it was, in fact, a case :-)
He’s out, dining with his dentist, when he hears from a waitress that an old painter suddenly changed his consolidated routine by ordering something very very unusual for him. Poirot finds it very interesting, and when he learns that the old man supposedly died falling down the stairs he starts investigating on his own, sure that he was murdered. 
Someone impersonated the old painter to make it look like he was still alive at dinner-time. His estranged twin brother had just died the week before, leaving no will, so with both brothers dead now the only relative to inherit it all would be their nephew George Lorrimer, a theatre guy. 

It wasn’t one of the best episodes, but Poirot is always Poirot, and Suchet’s Poirot is always worth it, just for him. 

I'm only sorry that I didn't understand the reference to the title, so I don't know what that means.