emacs: how to indent/unindent region of python code by 4 spaces?

Assuming that the variable python-indent is 4:

M-x python-shift-right (C-c >)
M-x python-shift-left  (C-c <)

indent-rigidly takes a prefix argument indicating how much to indent by, so C-u 42 C-x TAB indents by 42 columns, and since the default prefix argument is 4, C-u C-x TAB indents by 4 columns.

If you want to select the region again, do C-x C-x afterwards.

Use the indent-rigidly command with a numeric prefix.

C-u 4 M-x indent-rigidly to indent the region by four spaces, C-u -4 M-x indent-rigidly to remove four spaces.