"Throwing up 4s" is a phrase that is typically used to indicate that someone is showing disrespect or disdain towards someone or something. The "4" in this phrase refers to the number 4, which is a slang term for "death." Throwing up 4s can be interpreted as throwing up the sign of death, signifying that the person is saying that they wish someone or something would die or is dead.
This phrase is commonly used in rap music, particularly in some hip hop subculture, where the number 4 is associated with the Bloods gang and the letter B, which also represent "blood" and "brotherhood", which is considered as disrespectful to rival gang members. Therefore, the phrase can be used to indicate that someone is dissing or disrespecting a rival gang or individual.
It's worth noting that throwing up any hand signs or gestures such as 4s, or other gang signs can be considered offensive or disrespectful and it's not appropriate to use them in most social setting. It's important to be respectful of others and avoid using such phrases or symbols if you are not familiar with their context or meaning.