The volume control isn’t particularly linear across its whole travel. Up until around 10 o’clock, the gain ramps up quite quickly, then from 10 to around 3 o’clock, it’s pretty linear for around 20dB - I don’t have one in front of me at the moment to ‘count the dots’ as it were, but going off memory each dot from 10 to 3 should be (roughly) 4dB (with some ever so slight variation by up to 1dB). This is done to give a wide ‘fine tuning’ area. The sub gain in the app goes from -10dB to +10dB - this is the same in ‘Normal’ mode (1=-10dB, 5=0dB and 10=+10dB).
So if you’re going to +4, give a try moving the subwoofer volume control to the next mark down and experimenting around that area - your room and personal preference also comes into play (which is why we still allow full customisation of the settings after choosing your sub in the list, rather than locking you in). Sonically, I don’t see any issues doing either approach.