Here is an old thread that someone has the same question you do, some very informative posts near the end but the answer is still no unfortunately, you cant crossfire a 4650 with a 4200 igp.
"Unfortunatley" is the key word. Just want to keep an ear out cause this seems to be something that somebody will accidently figure out.
What puzzles me is why they build a crossfire board with only 1 PCI-E slot (and there are alot of them). That in itself lead me to believe the OB could be used. It could be an awesome mid-end link.