For words that have only 3 letters, there isn't a 4th letter. For example with the word “include”, there isn't any additional letters afterwards and therefore need to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have applied “addi” for “addict”. By https://indexedbookmarks.com/story18802532/5-essential-elements-for-bip39-explained