For words that only have 3 letters, there is no 4th letter. By way of example with the word “incorporate”, there aren't any additional letters Later on and so have to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you might have employed “addi” for https://canvas.instructure.com/eportfolios/3396498/ricardooxtv236/opening-the-world-of-bip39-a-comprehensive-overview-to-mnemonic-expressions-and-word-checklists