For words that have only three letters, there is absolutely no 4th letter. For example with the word “incorporate”, there won't be any additional letters Later on and therefore should be the word "insert". The word “addict” is from the BIP39 wordlist but you would've utilised “addi” for “addict”. BIP39 https://doctorbookmark.com/story19522770/5-essential-elements-for-bip39-explained