For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there isn't any extra letters afterwards and so have to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would've made use of “addi” https://setbookmarks.com/story18949270/5-simple-techniques-for-bip39-word-list-pdf