For words that have only 3 letters, there is no 4th letter. One example is with the word “insert”, there aren't any extra letters afterwards and thus must be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've applied “addi” for “addict”. A seed phrase, https://thesocialvibes.com/story4441778/fascination-about-invalid-bip39-phrase