First off, are you saying that the bar should buy them the 4th round? That
truly IS up to the managers and owners - I doubt you'll be able to convince
them otherwise, unless they have a fair amount of interaction with customers.
If you're talking about you, as an employee, buying them a round, I don't see
how management could complain. You're paying for the beer out of your pocket
- it's not like they're getting robbed. Every night that I bartend, I run a
personal comp tab along with my spill sheet. Sometimes it's empty, sometimes
there are 5-6 drinks on there, it just depends on who comes in and how
generous I'm feeling. My manager has never said a single thing about it -
he's just happy that I'm keeping honest track of EVERYTHING I pour, be it a
spill, a sell, or a comp.