For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there won't be any additional letters Later on and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you would've applied “addi” https://trackbookmark.com/story21263406/a-secret-weapon-for-bip39-words-to-numbers