The 146 started life as the HS681 - a VSTOL airlifter that was intended to support the P1154 (Supersonic Harrier II) force. It was powered by twin Pegasus with the efflux (one each side of each Peggy) being entrained by the flaps. To get the same effect required 4 small conventional jets. Again engine power, as such, had little to do with the choice of number of engines.