So experimenting with these settings I’m not sure that changing the tank to 3rd party, small coil made any difference to my HP performance (I have the 6kW EDLA, so essentially the same unit as your 4kW).
However, setting the quiet/more quiet/most quiet (I’ve settled on “more quiet”), as you mentioned, did makes a noticeable difference to the power consumption during DHW cycle. The good thing (imo), is that this mode only knocks down the top-end output of the unit (and so increases DHW COP) whilst not impacting CH performance. I will keep an eye on when temperatures get colder to see if this stops the unit from being able to return the tank to target temperature (mine is set to 46C, which the unit always overshoots by 1C 
)