For words that only have three letters, there is absolutely no 4th letter. For instance with the word “insert”, there isn't any more letters afterwards and therefore must be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would've made use of “addi” for “addict”. To https://bookmarkindexing.com/story19499024/fascination-about-bip39-english-words