For words that have only three letters, there isn't a 4th letter. As an example with the word “add”, there isn't any additional letters afterwards and thus should be the word "increase". The word “addict” is within the BIP39 wordlist but you might have utilized “addi” for “addict”. This extra https://canvas.instructure.com/eportfolios/3396498/ricardooxtv236/opening-the-world-of-bip39-a-comprehensive-overview-to-mnemonic-expressions-and-word-checklists