For words that only have three letters, there isn't a 4th letter. By way of example with the word “include”, there are no a lot more letters afterwards and so have to be the word "add". The word “addict” is inside the BIP39 wordlist however, you would've applied “addi” for https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and