I got lucky. I'd been getting the Error 4 message constantly for weeks, after getting Error 3 which would clear by turning off vehicle and restarting.
I was afraid I would need to get a new CD player (I still might!), but I tried a suggestion from this forum.
I got a credit card into the CD slot, and initially nothing happened. When I hit the eject button, it just switched to FM radio. Then I tried sliding the credit card in further, which I imagined to be on top of a CD, since I didn't see any CD edges to try to grab with a tweezers or long nosed pliers. After trying this a couple of times, and trying to push down slightly with the credit card, the Error 4 message changed to "5-1" and it started playing the first part of disk number 5. It also plays other CD's, ejects them, and reloads them (i.e. normal functioning). I hope it continues to work for a while.
Anyhow, the credit card trick is definitely worth trying!