Finally, I got the number 4 cylinder problem.

"The number 4 cylinder problem"? Either you're confused, or I am.

It sounds like you've never changed your spark plugs. They've got 120K miles on them. They're likely worn and fouled. This would lead to it running rich setting off sensor alarms and possibly the 'blue smoke' you saw.

It sounds like your confusing the issue of the number 4 spark plug being loose vs neglected spark plug maintenance. You never said the plugs were loose...were they?

Sounds like you've got some sensors to replace and codes to clear. Did you replace the plugs with platinum iridium plugs?

Yes, I've replaced all the 6 six spark plugs today. Except the #4 spark plug, the others look very good. And my friend told me not to change the others. He is a mechanic. After I changed the spark plugs, the blue smoke appeared. I'll let my friend do the check again next Monday. Thanks for the replies from you guys.

What they said ^
I had my dealer check mine @ around 10k mi/2 yrs, (thanks to threads here long ago) and sure enough, they found #4 was loose. As I recall, the did it under warranty/extended warranty.
A couple of years later, before the xtnded warranty expired, I asked them to do it again, and they declined, saying that they'd only do it once under warranty (going from memory here), and that 'once fixed, no need to recheck' very often.
After I reseached how much effort it took to do (I had NO idea - last plugs I changed was on an MGB!), I didn't blame them (much).
But sure proved to me it's a Real Thing, and worth checking!
(And might be worth adding it to my 'bucket list' of things I'm saving up to have done on it.)
Thanks for the reminder!