Correct option: 4) for is the most suitable preposition here,the correct sentence will be-Whom did we hire for the manager's post?
At :
At (to indicate a place)- There are a good number of people at the park.Generally at is used to denote smaller places.
At (to refer a time) — Meet me at 5 p.m. tomorrow.
At (indicate one’s activity)-John laughed at my acting in the play.
In :
In (to indicate a location)- I am in my friend’s place now. Generally used to denote bigger places.
In (used while doing something) — The tagline should be catchy in marketing a product.
In (to indicate opinion, belief, feeling, etc.)- I believe in hardworking.
In (specify day, month, season, year) — I prefer to do Maths in the morning.
The new academic session will commence in March.
In (to indicate color, shape and size) — This dress comes in four sizes.
For :
For (to indicate the reason or because of) — I am really happy for you.
For (to indicate the duration or time) — I attended the session for one year only.
For (specify the use of something) — She is preparing for her final exam.