For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “add”, there won't be any more letters afterwards and thus have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would have utilised https://bookmarksoflife.com/story4624797/new-step-by-step-map-for-bip39-english-words