Forget about what happens in the air -- If I were a FA, I'd be equally concerned about what happens when you get off the plane and get into your car (rental, own, or otherwise). They don't know what you had on the previous flight, what you had at the SkyClub, or what you had anywhere else. While 4 rum and cokes in 3 hours wouldn't cause me much harm, I know plenty of people that shouldn't be driving a car after that. I think the FA made an ok call here, and I applaud the OP for letting it be instead of challenging it (which would only reinforce the FA's decision, BTW).