For words that only have three letters, there's no 4th letter. One example is with the word “insert”, there aren't any far more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist but you would have used “addi” for “addict”. https://yourbookmarklist.com/story19072559/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator