For words that have only 3 letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters afterwards and therefore needs to be the word "add". The word “addict” is while in the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmarkinglife.com/story4348834/not-known-factual-statements-about-bip39-explained