Make the damned 4 hour drive anyway. These assholes are obviously finding it easy to lie to and put off a voice on the phone. Your being there is a lot harder to ignore.
Keep track of the time and your gas receipts and tolls, with paper proof if you can get it. So get the receipt from the gas pump, etc. Calculate your time, multiplied by the number of hours. Present that and the receipts and demand that they be paid in addition to your checks. Same if you have hotel and restaurant receipts from the trip.
Don't expect to be paid that, but have it with you, as it helps to impress upon them how much their inactivity has put you out.
Also get the name and phone number of the local (to them) TV investigative reporter. If they don't pay you, tell them you're calling the reporter. And then do it.
These people are incompetent and overworked at best (which means management needs to hire more people and build in more efficiency). At worst, they don't do their jobs.
They are counting on you just going away.
Don't let them get away with it.