Swearing in public places

My son has picked up the word “f***” and while he has no idea what it means he sometimes likes to use it interchangeably with “shut” with the unfortunate outcome that he has of late been saying/shouting “f*** your mouth” which is, needless to say, pretty awful to hear from a 4 year old.

When we’re by ourselves it’s one thing; when he says or shouts it say, on an airplane (as he did recently) or at the playground (as he did last night), I don’t know what to do. I really don’t want him to be the kid who exposes other children to swear words, and of course when confined on a plane you can’t simply go home. I haven’t really seen anything in the books about handling outbursts in public as opposed to when you’re in your own home.