Dart Herald
The change to turbo prop engines was probably simply due to the developing technology , so both the above observations are correct, the changes to the original configuration from four Alvis Leonides to two Darts resulted in an increase in performance, MTOW, and cabin size.
As this was a major design change to the airframe, there were significant changes to the empennage, amonst other structural re-designs, this resulted in a complex system of secondary controls, with spring, trim and gear tabs, with heavy balence weights, this was subject to prop wash and vibration, resulting in costly maintenance in materials and man hours towards the end of service, had the original design been to use two Darts, then I'm sure that the aeroplane would have enjoyed a longer life and cheaper operating costs.
Regards,
om15