For words that only have 3 letters, there is not any 4th letter. For example with the word “incorporate”, there isn't any extra letters afterwards and therefore must be the word "add". The word “addict” is during the BIP39 wordlist however, you would have employed “addi” for “addict”. You may https://bookmarkcork.com/story19757564/the-ultimate-guide-to-bip39-seed-phrase-github