For words that only have three letters, there's no 4th letter. One example is with the word “incorporate”, there aren't any much more letters Later on and thus needs to be the word "include". The word “addict” is in the BIP39 wordlist but you would have utilised “addi” for “addict”. https://madesocials.com/story4306308/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting