For words that have only three letters, there is no 4th letter. For instance with the word “add”, there are no more letters afterwards and therefore should be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have utilised “addi” for “addict”. This text clarifies https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and