For words that only have three letters, there is absolutely no 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and thus has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would've used “addi” for “addict”. To https://wisesocialsmedia.com/story5046403/top-guidelines-of-bip39-word-list-pdf