Dear Navi:

You wrote:

 Are these sentences correct:

1-He is as hard-working as ever, just changing his job. (Meaning: He is as hard-working as ever. He is just changing my job.)

I think you mean “his job.” Yes, that would be what it means.

2-John was really happy, just standing there listening to the waves.

This is fine.

3-John had a smile on his face, just standing there listening to the waves.

This is fine.

4-John was not doing anything, just standing there listening to the waves.

This is fine.

I do not like ‘1’. I think the “meaning” sentence is better. ‘1’ makes it sound that he is hard-working AS he is changing his job.  

No, remember that in English participial phrases are usually very literal. He is merely changing his job, not his work ethic.

I believe in spoken English, a full stop instead of a comma would solve the problem.

I am as creative as ever. (I am) Just changing my job. In written English, the second ‘I am’ has to be there.

Either way is fine.

I think ‘4’ also could use a full stop instead of the comma. It would then be equivalent to:

4a-John was not doing anything. He was just standing there listening to the waves.

Either one is fine and says the same thing.

I might sound as if I think I know what I am talking about. If I did not have strong doubts about what I was saying, I would not ask the question!!

I hope this helps.