‘But Poirot— why?’
This is possibly the most appropriate quotation I can choose to describe the book.
The Big Four has one of the silliest plots of Christie's books which is based on Poirot and Hastings engaging in international crime and espionage - much in the same way that Holmes and Watson do,just in a more believable way. Where Holmes is reserved enough to carry off disguises and under-cover work, Poirot's flamboyance has him stick out like a sore thumb all over the place, and Christie's attempts at making us believe that Hastings, or indeed anyone else, does not recognise him are just ludicrous.
The book was written in 1927 and is the first of Christie's novels that show her making an attempt at writing an espionage / crime thriller with an international setting. As ludicrous as this book is, it is still better than some of the other attempts that were to follow (I'm looking at you Passenger to Frankfurt!), but it is surprising that the book did not turn readers off Christie altogether.
Not only does the plot not hold any water, the writing is just unbelievably bad - culminating in an ending that is just ... wow, so ridiculous. Apparently, Dame Agatha got stuck and basically just hit the big red button. That is all I'm going to say about that (just in case someone really wants to read the book).
For the rest, ... Oh, boy. How many times can Hasting's get knocked out? Or knocked around by Poirot?
Seriously, for many parts of the book, I rooted for Hasting's shoving Poirot of a cliff.