For words that only have three letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there isn't any additional letters afterwards and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for “addict”. https://userbookmark.com/story18810515/an-unbiased-view-of-bip39-seed-phrase-github