For words that only have three letters, there's no 4th letter. For instance with the word “insert”, there aren't any more letters afterwards and thus must be the word "include". The word “addict” is while in the BIP39 wordlist however , you might have made use of “addi” for “addict”. https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and