For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “add”, there won't be any far more letters afterwards and so need to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have used https://bookmarkport.com/story21289378/invalid-bip39-phrase-things-to-know-before-you-buy