uh, its just away to call someone a 'hoe' without them fucking you up. if you dont know the connection of 304 and hoe, turn your screen upside down. ;-;
Billy: Hey, I like how you are reppin' the 304 on your new fitted cap.
Mac Daddy: Da Mac Daddy gotta keep it fresh son. You know how we do up here in DUB-V.
"304" is not a particular phrase people say, but instead just a cool cheap trick to call someone a hoe. You see, if you type "304" into a calculator (or something along those lines) and turn it upside down, you'll see that it will spell out hoe.
Let's say your friend is being a bully and types "304" into a calculator, she called you a hoe.