For words that only have three letters, there is absolutely no 4th letter. For instance with the word “include”, there aren't any extra letters afterwards and so should be the word "insert". The word “addict” is from the BIP39 wordlist but you might have made use of “addi” for “addict”. https://dmozbookmark.com/story19630856/the-definitive-guide-to-bip39-seed-phrase