For words that have only 3 letters, there is not any 4th letter. For instance with the word “add”, there won't be any much more letters afterwards and so needs to be the word "add". The word “addict” is while in the BIP39 wordlist however , you would've used “addi” https://olivebookmarks.com/story18910344/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting