For words that only have three letters, there is absolutely no 4th letter. One example is with the word “insert”, there are no extra letters Later on and therefore has to be the word "include". The word “addict” is within the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmarklinking.com/story5390577/ledger-seed-phrase-bip39-no-further-a-mystery