GWR four-cylinder arrangement?

Industrial and marine steam engines were almost universally compounds (or strictly speaking triple expansion engines), although of course they generally had condensers. The SS Titanic and her sisters had triple expansion engines driving the outer screws, with the exhaust from these combining to feed a turbine which powered the middle screw. These were said to have advantages over other contemporary vessels.
Going back to locomotives, I've always been intrigued by the idea of having a four cylinder compound locomotive with a Lord Nelson 45 degree crank angle arrangement. Assuming early exhaust release has advantages, and the LP cut-off is less than 50%, the "chuff" from the HP engine should hit the LP piston when it is more or less at the dead centre position. I'm thinking that this would give more power, but I don't know whether anyone ever built one.

 

Mallets are best suited to running at 15 to 25mph, i.e. for slogging up steep grades. At higher speeds power declines because of the unpredictable filling of the receiver. As such, Mallets would have been unsuitable for UP.

 

The higher level of training in France was linked to compounding; most French compounds had two reversers and two regulators! Even Chapelon's compounds were built this way until he worked out the optimum arrangement for each design. Compounding was usually reserved for tender passenger locomotives and some mixed traffic classes. Most French locomotives were not compounds!