For words that only have three letters, there isn't any 4th letter. By way of example with the word “include”, there are no much more letters afterwards and so must be the word "add". The word “addict” is within the BIP39 wordlist however you might have utilized “addi” for “addict”. https://singnalsocial.com/story4044670/invalid-bip39-phrase-things-to-know-before-you-buy