Baito Saki no Senpai ga Koukousei Janakatta Hanashi

Twenty-year-old college girl Yurari goes out to eat with Aoya, a senior at her part-time job, whom she thought was still a high school boy. But then she discovers he's actually four years older than her. Her impression of him is suddenly thrown into turmoil — as she wonders if there might be “something between them.”

(Source: Pixiv Sylph, translated)