For words that only have 3 letters, there is not any 4th letter. By way of example with the word “add”, there won't be any extra letters afterwards and therefore have to be the word "add". The word “addict” is within the BIP39 wordlist however, you would have made use https://apollobookmarks.com/story18790611/invalid-bip39-phrase-things-to-know-before-you-buy