For words that have only three letters, there is absolutely no 4th letter. For example with the word “incorporate”, there won't be any far more letters afterwards and thus must be the word "add". The word “addict” is in the BIP39 wordlist however you would have utilised “addi” for “addict”. https://bookmarkuse.com/story19299263/a-review-of-bip39-passphrase