L.A.
Just continue to tell him, "we do not use that word". "That is not an appropriate word." Then give him a "good word" to replace the bad word.
Let him know each time he uses an inappropriate word, he will get a time out and then do it..
Have you asked your pre-school how they handle these words? I am assuming they do the time out chair also..
1 Minute per his age.. 4 yo.. 4 minutes..