I agree with AC for the most part, but I think that 4yo IS old enough to learn it.
I may sound like a weirdo when I say this, but I believe in controlled and explained exposure to all things in life in order to teach children responsible use (except for pre-marital sex, but that's a religious issue and I do still believe in educating them about it, just not allowing them to experience it, lol!).
So basically, I swear in front of my children (picked up that bad habit in the Navy and I've toned it down a LOT since the pre-baby days). They know all of the bad words, what they mean and how they are used. They also know that they will be allowed to swear as much as they want when they are 18yo, but for now they are children and are not allowed to swear since they have to be adults to decide when it's inappropriate. This is the rule. I'm sure my oldest son's 18th b-day is going to be a doozy! lol! :) However, my older son is old enough to understand more, so now he knows that it's not a good idea to swear at a job interview or at work, in front of his girlfriend's parents (some day!), in church, around people that have children with them etc... And he knows why it's not a good idea and knows how it can damage the image of himself that he wishes projected to the world.
Oh and I also teach my children responsible knife and gun use beginning at age 7. My son is now 8, has been very responsible with his BB gun for over a year and has been told that he has earned a .22. He will get his new rifle sometime this summer. They should have exposure to these things from an early age in order to be practiced with the safe handling techniques by the time they reach adulthood. He is always supervised with his gun, always wears eye protection, always points the barrel at the ground, always unloads before storage and won't handle one of the bigger guns we have around the house, even with the trigger locks on, without one of us opening the chamber, removing the clip and handing it to him for inspection. He has demonstrated with a friend's .22 that he's capable of handling the "kick" and can handle the ammo responsibly. I'd say he's already more responsible with guns than many adult males I could find news articles about.
Again, the whole thing goes back to teaching responsible use. I DON'T think we should be sheltering our children from life. All that accomplishes is that they get sent out into the world completely clueless and innocent to get eaten alive.
Sorry to rant, but I hope you were able to discipher my point anyways. :)