For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any a lot more letters Later on and thus need to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have https://leftbookmarks.com/story19671034/ledger-seed-phrase-bip39-no-further-a-mystery