For words that have only three letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there won't be any much more letters afterwards and so should be the word "insert". The word “addict” is during the BIP39 wordlist however , you would have utilised “addi” https://friendlybookmark.com/story18705211/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting