For words that have only 3 letters, there's no 4th letter. For example with the word “incorporate”, there aren't any a lot more letters afterwards and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would've applied “addi” for “addict”. https://bookmark-media.com/story19563894/invalid-bip39-phrase-things-to-know-before-you-buy