For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “include”, there are no more letters afterwards and thus must be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have used “addi” for “addict”. https://bookmarkfavors.com/story4349335/ledger-seed-phrase-bip39-no-further-a-mystery