For words that have only 3 letters, there is no 4th letter. One example is with the word “include”, there won't be any extra letters afterwards and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however , you might have utilized “addi” for “addict”. https://tetrabookmarks.com/story18860738/a-review-of-bip39-phrase-list