For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there are no more letters Later on and so need to be the word "insert". The word “addict” is within the BIP39 wordlist but you might have used “addi” for https://mypresspage.com/story4240636/top-guidelines-of-bip39-word-list-pdf