I'd actually suggest that as the default behaviour (and is a good argument in favour of a dedicated function in textwrap - both suggested alternatives will blithely add whitespace to otherwise empty lines).

To handle the empty line requires either switching to an re.sub() based solution or adding a conditional expression:
    '\n'.join(((4 * ' ') + x if x else x) for x in s.splitlines())

I should probably also explicitly address the "why not textwrap.fill()?" alternative: because fill() does a lot more than simple indenting.