For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there isn't any more letters afterwards and thus has to be the word "insert". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. Meticulously overview https://gorillasocialwork.com/story20415271/a-review-of-bip39-phrase-list