I believe the name has to match, though I do not know for sure. But the United page says "All requests for missing mileage credit need to be made within one year of activity."
http://www.united.com/CMS/en-US/mark...lPartners.aspx

Were your own account one of the four? Did you pay the frequent flyer tax? That is usually a good indication of whether there were any miles from a car rental. I am assuming that you did not check soon after the rentals; but that would clearly have been a good idea.

I would guess you are out of luck. I hope you did not lose too many miles.