For words that have only three letters, there's no 4th letter. By way of example with the word “increase”, there aren't any far more letters Later on and so must be the word "include". The word “addict” is from the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://fellowfavorite.com/story20340586/getting-my-bip39-words-to-numbers-to-work