What is a four letter verb for obscene?

A four-letter verb that can mean "obscene" is "bawl," which refers to shouting or crying out loudly, often in a way that can be considered inappropriate or offensive. However, if you're looking for a verb that more directly conveys obscenity, "cuss" (to use profanity) might be the better choice. Both terms carry connotations of vulgarity in different contexts.