For words that have only three letters, there isn't any 4th letter. For example with the word “insert”, there are no more letters Later on and therefore need to be the word "add". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. - https://followbookmarks.com/story18898117/ledger-seed-phrase-bip39-no-further-a-mystery