For words that only have three letters, there is absolutely no 4th letter. Such as with the word “add”, there aren't any a lot more letters afterwards and thus need to be the word "add". The word “addict” is inside the BIP39 wordlist however you would have made use of https://bookmarkfriend.com/story18970473/new-step-by-step-map-for-bip39-english-words