For words that only have 3 letters, there is not any 4th letter. By way of example with the word “incorporate”, there won't be any much more letters afterwards and thus should be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have applied https://thebookmarkid.com/story18915916/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator