For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “include”, there are no more letters Later on and therefore has to be the word "include". The word “addict” is inside the BIP39 wordlist but you would've applied “addi” for https://agency-social.com/story4172375/a-secret-weapon-for-bip39-words-to-numbers