PEI,
The reason for the T-tail in the 146 has more to do with the fact that it was based on the HS681, which was a VSTOL military freighter originally designed to support the Harrier II (P1154) fleet. The T-tail came with the usual military aft cargo ramp. 146 also needed 4 engines as the 681 was powered by twin Pegasus and thus four jet effluxes to effectively cover the flaps.