For words that only have three letters, there is not any 4th letter. By way of example with the word “incorporate”, there are no additional letters Later on and thus has to be the word "add". The word “addict” is from the BIP39 wordlist but you might have utilized “addi” https://socialwebconsult.com/story4238626/getting-my-bip39-words-to-numbers-to-work