John.......
(a) rudder is sized to suit the aircraft. Doesn't matter whether you have two engines or four, the OEM is only going to give you enough rudder to meet the design standards and certification process requirements.
From my experience the 777-300ER is basically the same size as the 400 ( with similar total thrust ) but it has a much bigger rudder that deflects futher and has a tab. This from From the 777 FCOM 1
Yaw control is provided by a single rudder, which is almost the same height as the
vertical tail. The lower portion of the rudder has a hinged section (tab) that deflects
twice as far as the main rudder surface to provide additional yaw control authority.During takeoff, the rudder becomes aerodynamically effective at approximately 60
knots
I always understood that a big twin with very big donks on both sides needed a bigger rudder than a similar sized quad simply because of the huge thrust loss. Also the 400 only has 1 VMCA speed that caters for 1, 2 or 3 Engines out as far as I know.
take a look at the B52, it has a very small rudder.