On the unit I had, I found the settings from 1-4 much too dark to read in a completely dark room (e.g. a bedroom at night). When I compared it to my Kindle Fire, the darkest setting on the Fire was corresponding to a medium setting on the Paperwhite, while the Paperwhite's lightest setting was similar to the "medium" setting on the Fire.
Now, I understand that it does not need to be so bright, because in brightly lit rooms, the advantage of the eInk means you do not have to light it at all, but I found the lower settings much too dark for reading.
To make a long story short, for me the combination of Keyboard for summer/daytime reading and the (1st gen) Kindle Fire for nighttime reading and the dark season makes more sense, especially as the Fire was much sharper than the PW unit I received.