For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there are no additional letters afterwards and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you might have made use of “addi” for https://bookmarksoflife.com/story4430380/getting-my-bip39-words-to-numbers-to-work