[Perfectlap]

blinkwatt,
I was told that it's rare for all four to go at once and that replacing one in the same bank will not cause the other to fail. They are built to last 100K miles but my Boxster is on year 7 and I still haven't hit the halfway mark so I guess time is an issue as well. If you can pony up the $400-600 and have acess to a lift maybe its a good idea to do them all once but it doesn't sound like its really necessary. I think other forum members have changed just the one bad 02 sensor without any subsequent CELs.
p.s.
That $100 Actron OBDII reader is paying for itself. I saw the shop's pricing chart on the wall and they charge you $40 just to read and erase the CEL.