The slang “4” is another term from the police codes often adopted by the rap and urban community. The code “10-4” used by the police in transmissions and communications may mean “message received” or “affirmative.
In the vast rap vocabulary, 4 is often a simple affirmative reply. It is an acknowledgment of reception or positive feedback about a message. In a proper conversation, it might look like this:
“4, bro. I got your text about the reunion.”
“I’m all 4 about Eminem’s rap. He busts out rhymes with heart.”
The term is still widely used by several rappers. This slang has gained popularity in terms of texting and online message because of its easy use, similar to “yes.”