You need to give your child consequences for his actions.  First, take away there favorite toy or activity. If he continues to cuss, wash his mouth out with soap. I would threaten him first.  You need to show that you are the boss. Also, make sure that you do not repeat this language in front of him.  If you follow these measures, it should stop.  I talked about this on my show at www.blogtalkradio.com/totaltutor.

Hi there,
      First of all, you have to determine where your child heard those cuss words. Most likely it was a family member. Try to not cuss in front of your child and tell the rest of your family to do the same. The next thing you need to do is tell your son that saying those words is very bad and he will get punished the next time he says those words. If you have other children who cuss, make an example and yell at them when they cuss.
           I hope this helps.

I'm sorry to hear that your child thinks it's funny to cuss. I agree that you should attempt to figure out who or where your child has heard that kind of language and hopefully be able to put an end to it. It may be from a family member, friends, even TV... I think it's really important to not laugh at this behavior, your child might take the attention as positive reinforcement.

Below is a great article with more in-depth advice. Hope this helps!

Hi again,

I found some additional resources that could help...

Most kids think its OK to cuss because family members or friends cuss alot. what i did with my son i made up funny words and say funny words when i got hurt my kid learned not to say cuss words but funny random words

Hi feliciaky,
It's great that you contacted Just Ask on education.com concerning this matter.  First concern is the source of the words, as parents, we have to take responsibility for what our children are watching on tv or the content of movies.  Next, we should refrain from using foul language in front of our children.  Determine the source of the words, and put a block on it.  Second, why does your child think that it's funny, that's an indication to me that he is aware of what he is saying and has been encouraged to continue repeating these words, so try not to laugh when you hear him.  You need to remind him that: "That's ugly, not funny and it's not nice to hear out of a precious mouth like yours".  "Don't say that again, people won't like it, so it's a monster word".  Teach him not to use the monster words.  I wouldn't punish a 4 year old for this, he got it from a source, but teach him that it's ugly and he's not, so don't act ugly.

Good luck!

Here is an article that you may find interesting!