Bill

If your Sony receiver is design to run specifically at 4ohm(which is doubtful) then you shouldn't hear any sound quality loss, but as I stated earlier though... the lower the impedance you dropped, the higher the THD going to rise, and damping factor will get slash in half or more, and the hotter your receiver will become, all these might not mean anything to you, but that's the nature of Class A and A/B design.

If I were you I would run it at 8ohm.