For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any more letters afterwards and so has to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would have used “addi” for https://webookmarks.com/story4329011/the-2-minute-rule-for-bip39-all-words