For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there are no more letters Later on and therefore have to be the word "add". The word “addict” is during the BIP39 wordlist however, you might have made use https://canvas.instructure.com/eportfolios/3396498/ricardooxtv236/opening-the-world-of-bip39-a-comprehensive-overview-to-mnemonic-expressions-and-word-checklists