They try to show effect of 2x and 4x lower speaker impedance - so for 4 ohm speaker two other traces are 2ohm and 1ohm. Not only that 1000ASP is not rated for 1ohm but traces should not meet at 1kHz since it is always ratio of speaker impedance to output impedance. IMHO they screwed it up. It is done properly on 200ASC datasheet page 14
http://www.icepower.bang-olufsen.com/files/solutions/200asc.pdf
I would just ignore two other curves and use uppermost one (red) on 4 ohm chart.
http://www.icepower.bang-olufsen.com/files/solutions/200asc.pdf
I would just ignore two other curves and use uppermost one (red) on 4 ohm chart.