For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any more letters Later on and therefore needs to be the word "incorporate". The word “addict” is from the BIP39 wordlist but you would have utilized “addi” for “addict”. https://thebookpage.com/story4177210/the-5-second-trick-for-ledger-seed-phrase-bip39