For words that only have 3 letters, there's no 4th letter. As an example with the word “incorporate”, there won't be any extra letters Later on and thus have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would have employed “addi” for “addict”. https://bookmarkinglive.com/story19839807/fascination-about-invalid-bip39-phrase