For words that only have 3 letters, there's no 4th letter. For instance with the word “increase”, there aren't any extra letters Later on and thus need to be the word "include". The word “addict” is within the BIP39 wordlist however, you would have applied “addi” for “addict”. In this https://socialrator.com/story9502088/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting