I was in a restaurant with a similar story like yours, the customers made a big fuzz about having 3 beers when the bill said 4, he also stated he never drink more than 3 beers in an evening. In his case they let him off paying 3 beers, and I just by accident was there a few days later when he came back and the manager asked him to follow. Apparently they had it clear on the CCTV that he had ordered 4 beers that evening. He had to pay for 4 only drinking 3 that evening. I asked the manager sinca I know him a bit, and he say he is a regular customer, and just said we was so sorry he must have forgotten one because he usually only have 3.
I do forget how many beers I drink sometimes, and if they take the bottles away like they do on some places I just kindly ask them to leave them so I can keep count... My wife taught me to do that.
She left off in a bit of rage and almost in tears because she lost her face, being wrong or at least corrected by the boss. She was probably sure she was correct otherwise my guess is she would have corrected it directly as they usually do. If they later can confirm you had 4 she probably has to pay for one out of her own pocket. And that remind me of another story, we went to a Thai restaurant and we ordered a big plate of Khao pad poo. Fried rice with crab, and when severed we noticed it seamed wrong and called the guy serving us, and he said he was sorry since he thought Khao pad Moo, Fried rice with pork. So he took it back and came back a few minutes with the correct dish, He was very helpful and during he was helping us filling classes of beers and such, one of my Thai friends asked what happens with the wrong order, and the servant say he has to pay for it with own money, and he can eat it hungry or not no staff discount given.
We gave him tip under the pay slip thingy enough to purchase a lot of Fried rice pork.