For words that have only three letters, there's no 4th letter. As an example with the word “add”, there isn't any a lot more letters Later on and thus must be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://getidealist.com/story20550163/facts-about-bip39-seed-phrase-github-revealed