You said yourself in your scenario: the four of them drink equal amounts, and run up a tab of $50 dollars in total. That means that if all four ask for their tab simultaneously, they'd all get a tab of $12.50.
ETA: OK, I'll retell your metaphor as I think it really is.
Four friends - an American, a Belgian, a Czech and a German - sit at a bar and drink equal amounts. The American leaves early, waves with his hand and says to the bartender: "write it on my tab, whatever the price is". The other three empty their glasses and ask the bartender for the bill. According to the bartender, the tab has run up to $50, so he asks them $12.50 each.
The Belgian says: for that price, I could have gotten a real Trappist next door.
The Czech says: you realize that the price of hops has gone down dramatically, don't you?
The German says: this American beer you served isn't even real beer, ever heard of the Reinheitsgebot?
So the bartender says: OK, I'm fine if you all pay me $10.
And so they do.
And the bartender writes $20 on the tab of the American, who just pays it without questioning where the amount came from.