For words that only have three letters, there isn't any 4th letter. By way of example with the word “insert”, there are no additional letters afterwards and therefore must be the word "add". The word “addict” is while in the BIP39 wordlist however, you might have used “addi” for “addict”. https://geniusbookmarks.com/story19135909/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator