For words that have only three letters, there is no 4th letter. For example with the word “incorporate”, there aren't any a lot more letters afterwards and thus should be the word "add". The word “addict” is while in the BIP39 wordlist however you would've made use of “addi” for https://bookmarksoflife.com/story4623938/considerations-to-know-about-bip39-word-list-pdf