For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any far more letters Later on and thus should be the word "include". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. https://ztndz.com/story22140579/ledger-seed-phrase-bip39-no-further-a-mystery