Well 4 ohms means less resistance this means the speakers will not "resist" the signal from the speaker outpts of your amp as much as an 8 ohm speaker would, so this may mean you get a more detailed sound, this just a guess though 
. My system is a 4 ohm capable tube amp and 4ohm tekton ob4.5's I've been extremely happy with this set-up
The impedance value is 1) Nominal, but not fully inclusive of the actual load. Impedance varies by input and frequency. Klipschorns for example may be listed at a "nominal" 4 ohms, but audio magazines measured a very complex curve ranging (IIRC) from 2.7 to 32 ohms. 2) Speaker impedance has nothing to do with quality or potential...it's just the load, most of which is the woofer and midrange. The tweeter load isn't seen to a great degree. 1,2,3,4,5,6,7,8.... that is irrelevant to the sound quality. 3) Some speakers are 8 ohms and a monster to push (likeATC dynamics, electrostats or ribbons for example) or maybe 4-6 ohms and a breeze (line arrays/horns.)
