If 4.2 (or 4.1) is really the annual coefficient of performance (turn 1kWh electric energy into 4.2kWh heat energy) then this is a very good value.

Because heating water requires higher temperature differences (usually heat pumps get more inefficient then). And with an "ordinary" (propane) heat pump you get such high numbers only in summer time (>=20°C) and for water temperatures of max 50°C.