For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there isn't any extra letters afterwards and therefore should be the word "add". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. Mnemonic words examples https://bookmarkextent.com/story20486799/considerations-to-know-about-bip39-word-list-pdf