For words that only have three letters, there is not any 4th letter. As an example with the word “insert”, there won't be any extra letters afterwards and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would've employed “addi” for “addict”. https://pageoftoday.com/story4326274/ledger-seed-phrase-bip39-no-further-a-mystery