For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there are no more letters afterwards and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for “addict”. BIP39 and forty https://exactlybookmarks.com/story18988055/5-essential-elements-for-bip39-explained