For words that have only three letters, there isn't any 4th letter. For instance with the word “insert”, there are no extra letters afterwards and thus has to be the word "include". The word “addict” is within the BIP39 wordlist but you would have used “addi” for “addict”. This information https://bookmark-vip.com/story19096555/bip39-seed-phrase-mnemonics-generator-an-overview