had been working/ was working/worked

[ROFTOK]

1 He had been working as a manager for 1 year since he left our town.
2 He had been working as a manager for 1 year since he had left our town.
3 He was working as a manager for 1 year since he had left/had left our town.
4 He worked as a manager for 1 year since he had left/had left our town.

Which option are plain wrong if any?

 

[5jj]

None of them is as natural as "He worked as a manager for a year after he left Stockport" (or whatever the name of 'our town' is).

 

[ROFTOK]

I am sure you are right. But it doesn't make them wrong. Here is an example from a dictionary.

He had been composing music since he was ten years old.

 

[5jj]

Example sentences from dictionaries do not give the full context. Neither do your four sentences.