For words that only have three letters, there's no 4th letter. As an example with the word “add”, there won't be any much more letters afterwards and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however , you might have made use of “addi” https://ztndz.com/story21927850/top-latest-five-bip39-words-urban-news