For words that have only 3 letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any more letters Later on and therefore has to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. https://hotbookmarkings.com/story19046212/bip39-seed-phrase-fundamentals-explained