Why are there 4 beats to a bar ?
There aren't always. ? It's mostly a number divisible by two (Hence 2/2 timing, which isn't uncommon) It's a convention that most music uses to make the majority of time signatures relatively easy to follow. As the denominator is a division of a whole note and the easiest devisor to use is 2, we end up with 2, 4, 8, 16, 32 being the easiest to use.
Some composers
do use what are called irrational measures with "odd" divisors, these are often in multiples of three to make tuplets easier to notate. Henry Cowell used everything from 1 to 9 in
Fabric, and Brian Ferneyhough, Thomas Adès and John Pickard all commonly use irrational measures.
Note: Dweezil Zappa used to write parts in 1/1 timing to make some of his more complex passages easier to follow. I don't know if he still does though.
