For words that only have three letters, there is not any 4th letter. One example is with the word “insert”, there aren't any much more letters Later on and thus has to be the word "incorporate". The word “addict” is during the BIP39 wordlist but you would have used “addi” https://icelisting.com/story19833655/5-simple-techniques-for-bip39-word-list-pdf