You are of course correct that there were only three ships not four in the class. Also that in terms of engineering the Cunard ships were superior being driven by high pressure steam turbines.
The White Star ships were, however, larger and in civilian services more luxurious. White Star deliberately opted for a hybrid system of reciprocating engines driving the outboard propellers and a low-pressure turbine driving the center propeller. This produced less vibration for the luxurious passenger experience upon which White Star prided itself.
Actually the opposite
Turbines produce less vibration than reciprocating engines.
Turbines just spin.
It was thought that reciprocating engines were more reliable and less costly to repair and did not require such massive high pressure Plumbing Engineering

