For words that only have 3 letters, there is no 4th letter. For instance with the word “incorporate”, there are no far more letters afterwards and thus need to be the word "increase". The word “addict” is in the BIP39 wordlist however you would've employed “addi” for “addict”. In this https://bookmarkvids.com/story20097876/5-simple-techniques-for-bip39-word-list-pdf