For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any additional letters afterwards and so need to be the word "increase". The word “addict” is in the BIP39 wordlist however, you might have used “addi” for “addict”. Unlock https://bookmarkspiral.com/story19055491/bip39-seed-phrase-mnemonics-generator-an-overview