For words that have only three letters, there is not any 4th letter. One example is with the word “insert”, there are no much more letters afterwards and therefore have to be the word "add". The word “addict” is inside the BIP39 wordlist but you would've utilised “addi” for “addict”. https://expressbookmark.com/story18865139/the-5-second-trick-for-ledger-seed-phrase-bip39