There is a guy in our AA group that, coincidently, has 4 years of sobriety. He sponsors others and helps out quite a bit. The problem is that there is some gossip going around that he doesn’t actually have 4 years of sobriety because on some recent lifestyle choices. The gossip is not healthy for him, his sponsees, newcomers, or the group for that matter.
My opinion, for what it’s worth, is that your 4 years of sobriety is intact. There was no intention to drink and you thought you were drinking a non-alcoholic beer. If that is truly all there is to it, then there is no need to blow it out of proportion - eg. setting yourself up for relapse, quitting AA.
However, I would be open to the group about it. You don’t need people rolling their eyes, questioning your integrity every time you mention your sobriety date.
By the way, if someone brought this situation to an AA meeting that I attended, I wouldn't consider it to be a big deal, just an honest mistake.