For words that only have 3 letters, there's no 4th letter. For instance with the word “increase”, there aren't any more letters afterwards and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have applied “addi” for “addict”. To test to understand https://push2bookmark.com/story18955735/bip39-seed-phrase-fundamentals-explained