You can find your common ancestor because your family tree is probably complete at least a few generations back and so are the other people's. We'll have to hope that Jane's strongest match knows his or her family tree.
And again, 4 matches with greater than 40 centimorgans is a low number. Look to see how many you have. I'll bet you a nickel you have more than 4.
I'm confident they'll be able to find the common ancestors for this couple and trace their connections to identify them. Genealogists use more tools than DNA and other people's family trees to research their family history. Birth and death certificates, census records, military, school, probate and other records are also research tools.