It's impossible to expect the FA to know your level of intoxication (or lack there of) on a flight, the only think they can do is judge by the amount they personally served you. However its quite possible that before getting on the plane and consuming the 3 drinks - you were at the bar drinking before the flight. It doesn't sound like this was the case here, but it happens A LOT.

Most establishments have specific guidelines around # of drinks consumed within a specific period of time, and typically requires a waiter/waitress to notify management before serving drinks over that limit.

Seems like the FA was giving you a 'heads up', if the FA refused the 4th drink completely then you might make the argument that she could have been lazy. She did in fact serve and then provided the notice that it was the last drink. If you really wanted a 5th drink you could simply ask and then let her make a judgment call then.