For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “include”, there aren't any additional letters Later on and so have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've made use of https://bookmarkoffire.com/story18728684/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool