The digit 4 on a keycap. It stands for four, the fourth item, or the word 'for' in playful spelling.
People number lists with 4️⃣ and write things like 'this one is 4️⃣ you'. It also counts a group of four, like a team for doubles.
In Chinese, Japanese and Korean, the word for four sounds like the word for death, so some people there avoid the number.
Facts
Keycap: 4 4️⃣ is an emoji in the symbols category, under keycap. It is one of the early emoji from Japanese phones, standardized in Unicode 6.0 (2010). On GitHub, Slack and other apps you can type it as :four: or :keycap_4:. People also look for it with words like “4”, “four”, or “keycap”.
Timeline
2010Added to the emoji standard (Unicode 6.0)
Design history
Google (Noto)
2015
2017
2021
Google Noto Emoji images: Apache 2.0. Twemoji graphics by X Corp and contributors: CC-BY 4.0.