For words that have only three letters, there is no 4th letter. For instance with the word “include”, there won't be any much more letters afterwards and therefore should be the word "insert". The word “addict” is during the BIP39 wordlist however , you would have made use of “addi” https://socialwebconsult.com/story4357915/considerations-to-know-about-bip39-word-list-pdf