python about PEP8, indent

Question

if you use spaces, only use 4 per line. why not 1 space?

Answer 1

Because readability matters. When you have complex code with many nested levels, it is easy to loose track which lines belong together in the same block, when the indentation does not guide your eyesight.
https://realpython.com/JUMP_LINK__&&__python__&&__JUMP_LINK-pep8/

Answer 2

Because a program that uses 4 spaces for indentation is much more readable than one that uses 1 space. In fact, a program that uses 1 space for indentation deserves to be deleted as soon as it is made.