For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “include”, there isn't any extra letters Later on and therefore must be the word "insert". The word “addict” is in the BIP39 wordlist however, you would've applied “addi” for “addict”. BIP39 https://altbookmark.com/story20784143/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator