For words that have only three letters, there is no 4th letter. One example is with the word “insert”, there won't be any more letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. https://thesocialvibes.com/story4332753/little-known-facts-about-bip39-seed-phrase-list