For words that only have three letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any a lot more letters Later on and therefore must be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would've utilised “addi” for https://thefairlist.com/story9717577/not-known-factual-statements-about-bip39-explained