Meaning of 五 in A: 放肆！B: 你爹放五!
It's a funny one! 

As 5 (五) is bigger than 4 (肆)， it could imply B is one higher generation than A. That's why B said 你爹（your father, 放五）， implying "I am your father"(as you are "放肆" and I am "放五"！). This is like saying "我是你爹，放五！"

I really love this joke! Couldn't help laughing...

PS： better put a comma in the middle "你爹，放五！"