Most high-quality loudspeakers are 4-Ohms

NO.....

Any 4 ohm little monitors out there that can out do the 15 ohm Rogers LS3/5.♫

I think in a lot of cases they could design a lot of those 4Ω speakers to have only an 8Ω load.That may take a lot more engineering,plus better parts in crossovers,drivers,etc.The result,higher cost.When you look through buyers guides,you will find a lot of pricey high end speakers that are 8Ω.With some manufacturers their top flagship will be 8Ω speakers,and
their cheaper models are 4Ω.And also the opposite.I have good and bad that are both.Sometimes a new version will come out with higher impedance.HT 2009 buyers guide examples.(Snell Illusion,$50,000,8Ω)( Snell C7,$9,000,4Ω)(Totem Arro$1,350,4Ω)( Totem Forrest,$3,495,8Ω)They vary all different ways.It's hard to say that 4Ω speakers are the best.Have you compared them to all the over $50,000 8Ω speakers to see if they really are better?♫