For words that only have three letters, there is not any 4th letter. One example is with the word “insert”, there isn't any additional letters Later on and thus has to be the word "add". The word “addict” is from the BIP39 wordlist however , you would have applied “addi” https://agency-social.com/story4010089/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator