For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there are no extra letters Later on and therefore have to be the word "add". The word “addict” is inside the BIP39 wordlist however you would have utilised “addi” for “addict”. https://bookmarkgenius.com/story18910483/facts-about-bip39-seed-phrase-word-list-revealed