For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “include”, there won't be any additional letters Later on and therefore has to be the word "add". The word “addict” is inside the BIP39 wordlist however you would've utilized “addi” for “addict”. https://bookmarkstown.com/story19832471/ledger-seed-phrase-bip39-no-further-a-mystery