impedance

I accidentally purchased a $400 Center Channel speaker with 4 ohm impedance, forgetting my Denon AVR-X4500H only supports 6 and 8 ohm impedance. How will this negatively affect my audio quality? Is there a a piece of hardware I can add to add impedance? Hopefully one the goes on the speaker wire...

Hi, I'm rebuilding a 1970's stereo cabinet with modern components, It will be set up as a 2.1 system, I currently have qty (2) Sononance MAG6Rs for my mids and tweeters (8 ohms). I have a receiver that shows 6-16 ohms resistance (Denon AVR-1913) and the subwoofer driver I'm looking at...