For words that have only three letters, there isn't a 4th letter. One example is with the word “incorporate”, there are no additional letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist but you would have made use of “addi” https://madesocials.com/story4306555/a-secret-weapon-for-bip39-words-to-numbers