After Many, Many attempts to correct/clear the dreaded Error 4 code on my RX330 and out of desperation, I forced a CC in the CD slot with the key on, I could feel a movement on the CC, I left it there for 3-5 seconds, and when I pulled the CC out, the CD player started playing the CD.
I ejected all the CDs (6) and put 1 back in, it played, I put in another CD and it played too.
I shuttled between disc 1 and disc 2, No Issues.
I only have 2 CDs in the 6 disc changer, and it works Great and NO ERROR 4 any longer.
I don't kno if this will work for everyone, everytime, but it worked for me. Just a thought.