For words that only have 3 letters, there isn't any 4th letter. As an example with the word “include”, there are no additional letters afterwards and so have to be the word "increase". The word “addict” is in the BIP39 wordlist however , you would've utilised “addi” for “addict”. bip39-phrase.com https://fatallisto.com/story8548228/the-invalid-bip39-phrase-diaries