For words that have only 3 letters, there's no 4th letter. One example is with the word “incorporate”, there aren't any a lot more letters afterwards and therefore need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would have employed “addi” for “addict”. https://social4geek.com/story4729836/the-invalid-bip39-phrase-diaries