The word that represents the subject of this sentence is 4) I.
The subject is one of the most basic parts of a sentence and it generally refers to a person or thing that is performing an action. Moreover, on some occasions, the subject is just being something. In this specific sentence, the subject is a pronoun ("I") and it is performing an action, that is "inviting". Furthermore, it is a simple subject because it is formed with just one word and no modifiers.