I'm using the latest version of PIL (1.7.7) and even though a patch adding support for the group4 encoding was provided on the Image-SIG mailing in 2007 (http://mail.python.org/pipermail/image-sig/2007-October/004642.html) the current version still doesn't seem to handle group4:
~/.virtualenvs/tiff2pdf/local/lib/python2.7/site-packages/PIL/Image.pyc in _getdecoder(mode, decoder_name, args, extra)
383 return apply(decoder, (mode,) + args + extra)
384 except AttributeError:
--> 385 raise IOError("decoder %s not available" % decoder_name)
387 def _getencoder(mode, encoder_name, args, extra=()):
IOError: decoder group4 not available
It seems that PythonWare doesn't actively maintain PIL any longer. Is anyone else working on it?