For words that only have three letters, there's no 4th letter. For instance with the word “increase”, there aren't any more letters afterwards and so must be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have applied “addi” for “addict”. By pursuing these guidelines, https://social-galaxy.com/story4257164/5-essential-elements-for-bip39-explained