I don't think it's fair for those giving UA a hard time about this. Award mileage requirements are clearly posted on their web site so this is an obvious erroneous error. Revenue tickets are a different story since prices constantly change and there's no set price.
I feel it's naive to believe someone is entitled to this ticket for only 4 miles and think UA has been nice enough to honor even tickets through July 21. While I'm all about getting things for free I think if you want to make complains to the DOT you are really just trying to take advantage of something you shouldn't have received anyway. It's like finding a million dollars in your account and shocked when the bank wants it back. Better off sticking with mistake fares since those will be honored. :-D
However, if you did already depart on your trip, congrats!