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Oops, it should have been:
>>> re.compile(r"(\ba*){4}")
And now the error is consistent (now tested in Python 2.4
instead of 2.3):
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/sre.py", line 180, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.4/sre.py", line 227, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

So it seems that {m} operator does not like potentially
empty matches.