Sorry to hear this.
I suppose it is the 'job' that you quoted for but then what would happen if you quoted a job which took nstead of 4? Do you charge less or the same? Maybe that is where they are coming from.
I feel that they should pay the agreed amount but on the flip side I've had work done where it's taken less time & the contractor has charged me less which impressed me & made me want to use them again and also recommend them to others.
Granted for you it's only a couple of hours so they should pay the full amount in your case.