The "4x" is arbitrary. The discount in general not.

With "arbitrary" I mean there is no scientific reason why exactly this number is the correct one. 

 It isn't arbitrary, it's the smallest whole number where we cross that threshold of making inputs cheaper 

 Sounds arbitrary to me. Why not use a decimal number ;)

Ok, TIL. Will read up on the details ... 

 You don't want to use floats for money