He plays piano well.

[navi tasan]

1) He plays good piano.
2) He plays a good piano.
3) He plays piano well.
4) He plays the piano well.

I think #2 is ambiguous. It could mean that the piano he plays is good or that he plays the piano proficiently.
I use #3 or #4 for that meaning. Are #1 and #2 standard for that meaning? Are they acceptable in formal English for that meaning?

 

[Skrej]

1) He plays good piano.
2) He plays a good piano.
3) He plays piano well.
4) He plays the piano well.

I think #2 is ambiguous.It could mean that the piano he plays is good or that he plays the piano proficiently.

Correct, but see comments on #2 below.

I use #3 or #4 for that meaning. Are #1 and #2 standard for that meaning? Are they acceptable in formal English for that meaning?

#1 - Not in formal English, but you will likely hear it in everyday vernacular.
#2 - Even less formal. You're more likely to hear this variation with 'mean' (slang for very proficient, excellent, talented, etc. ) instead of 'good'. I'd consider this usage slang.