For words that only have 3 letters, there is not any 4th letter. One example is with the word “insert”, there won't be any a lot more letters afterwards and therefore need to be the word "add". The word “addict” is while in the BIP39 wordlist but you would've applied https://bookmarkgenius.com/story18717769/the-2-minute-rule-for-bip39-all-words