For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “include”, there won't be any additional letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however you would've utilized “addi” for “addict”. https://kingslists.com/story20031107/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting