For words that only have three letters, there isn't a 4th letter. As an example with the word “incorporate”, there isn't any more letters afterwards and so has to be the word "insert". The word “addict” is in the BIP39 wordlist but you would have employed “addi” for “addict”. Try https://bookmarkoffire.com/story19529219/5-essential-elements-for-bip39-explained