For words that have only three letters, there is absolutely no 4th letter. For instance with the word “add”, there are no extra letters Later on and thus needs to be the word "include". The word “addict” is in the BIP39 wordlist however , you would've used “addi” for “addict”. https://bookmarking1.com/story19025404/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator