Cracking 4s is a dissed aimed at any gang that rep the number 4. Gang members who rep the number 4 often throw up 4 fingers (excluding the thumb) up. The cracking 4 symbol is often done by raising 4 fingers and then proceeding to bend all of them.
In Chicago, the Four Corner Hustlers use 4 as of their symbols, their enemies crack them as a show of disrespect. In Tay-K's The Race, he raps, "Eli blast crackin' 4s like a vase." Eli is cracking 4s for another gang, but the meaning is the same.