For words that only have three letters, there is not any 4th letter. As an example with the word “include”, there won't be any extra letters Later on and therefore should be the word "insert". The word “addict” is while in the BIP39 wordlist but you would've made use of https://squareblogs.net/edelinepge/h1-b-recognizing-bip39-a-comprehensive-overview-to-mnemonic-expressions-and