When my daughter was 4 I asked her to say all the bad words she knew. She started with the not so bad ones and ended with all the worst ones.
I guess the other kids in her kindergarten had older siblings, so she knew them all already. But most importantly, she also knew when to use them and when not to use them. :)
I remember getting in trouble in preschool for wondering aloud how you spell "bitch." Timeout wasn't even the worst part; they ruined all my markers ;(