People from a different country.
Person 1: "That guy speaks as if he has mental disabilities."
Person 2: "Must be a 4ner"
An alternative form of "foreigner", usually said with some form of negative connotation. It can refer to anyone outside of your country, but is most commonly accepted as applying to anyone from a non-English speaking country.
1: Lmao look at xox h0z, what a fucking 4ner.
2: I can't understand a word my math teacher says, he's a damn 4ner.
3: I tried calling customer service to get my internet fixed, but everyone there were all 4ners