For words that only have three letters, there's no 4th letter. As an example with the word “increase”, there aren't any more letters Later on and thus needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. bip39-phrase.com https://bookmarkvids.com/story20077703/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool