For words that have only three letters, there isn't a 4th letter. One example is with the word “incorporate”, there aren't any far more letters afterwards and therefore need to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you might have applied “addi” for “addict”. https://bookmarklayer.com/story18842832/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool