For words that only have three letters, there's no 4th letter. One example is with the word “insert”, there are no a lot more letters afterwards and so should be the word "include". The word “addict” is while in the BIP39 wordlist however , you would've applied “addi” for “addict”. https://esocialmall.com/story4345752/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator