For words that only have 3 letters, there isn't any 4th letter. As an example with the word “include”, there are no far more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would've utilised “addi” for “addict”. https://prbookmarkingwebsites.com/story22298626/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase