For words that only have 3 letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any much more letters Later on and so have to be the word "add". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. https://setbookmarks.com/story19074259/facts-about-bip39-seed-phrase-word-list-revealed