For words that only have 3 letters, there isn't a 4th letter. One example is with the word “insert”, there isn't any more letters Later on and therefore have to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you would have used “addi” for “addict”. https://sirketlist.com/story20470202/top-guidelines-of-bip39-word-list-pdf