For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “include”, there are no extra letters afterwards and therefore has to be the word "add". The word “addict” is in the BIP39 wordlist but you would have used “addi” for “addict”. https://xyzbookmarks.com/story18661851/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39