Can you prove that you only had 4 pints. The barman and the taxi driver would be needed for this. I think a complaint of false arrest would do it. You can make life unconfortable for the gardai. But the downside of that is that they will be after you forever. Is that worth it?
I would just like to make one comment on your first post, I thought your story was over the top originally and that you were actually drunk. If you were not than what happened to you is really out of order. Gardai getting their kicks, they do like the power they have. If you're brave enough to stand up to that then fair play to you.
Having 4 pints is not going to be seen as being drunk. I was in the High Court once and the judge (a well known drinker) took umbridge at the opposing barrister when he outlined the amount of drink the plaintiff had taken (all day drinking) so you never know with judges.
Having said that, judges are on the sides of the gardai in general, recent case in Donegal where judge made comments to the contrary being a rare exception.
It's a terrible thing to say of the law that in a case where you are in the right and those who profess to be uphholders of the law abuse that position, then the law is of no use to one. But that would apply to lots of Irish society.