For words that only have three letters, there's no 4th letter. One example is with the word “incorporate”, there won't be any much more letters afterwards and thus need to be the word "include". The word “addict” is in the BIP39 wordlist however you would have applied “addi” for “addict”. https://social-galaxy.com/story4293965/invalid-bip39-phrase-things-to-know-before-you-buy