I'm reading a short story and I wondered what 五 means here but I guess it's 'slang'.
B retorted back: 你爹放五!
(Sorry for not putting any research result, can't find any, I only know that 五 is sometimes used to replaced 我 or 无).
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！"
Five is bigger than four. 肆 is 4.