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1- "only" modifies "has been". It emphasizes more on the status of the action.
2- "only" modifies "for a week" where the duration is stressed.
3- "only" modifies "a week". It tells us the exact days he has been there.
4- "only" modifies "a week" or "for a week". Time advervial receives more emphasis.
shades of meaning 
Linear Mode
