1. She is a stewardess with Korean Air.

2. She is a stewardess at Korean Air.
3. She is a stewardess in Korean Air.
4. She is a stewardess for Korean Air.
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Can we use other prepositions instead of 'with' as in Sentence 1?

I'd use 1 and 4 ... and maybe 2!

The current term is "flight attendant" since people who do that job are not always women anymore.