For words that only have three letters, there's no 4th letter. By way of example with the word “incorporate”, there are no additional letters afterwards and so needs to be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. https://bbsocialclub.com/story20669763/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39