For words that have only three letters, there is no 4th letter. One example is with the word “insert”, there isn't any a lot more letters Later on and thus must be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have used “addi” https://dirstop.com/story21855787/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase