For words that have only 3 letters, there's no 4th letter. As an example with the word “insert”, there are no far more letters afterwards and thus must be the word "include". The word “addict” is while in the BIP39 wordlist however, you would've applied “addi” for “addict”. This information http://andersonsaeu988.iamarrows.com/from-mnemonics-to-safety-and-security-recognizing-bip39-passphrases-and-their-significance-in-copyright