For words that only have 3 letters, there is no 4th letter. For example with the word “add”, there are no extra letters Later on and therefore has to be the word "include". The word “addict” is from the BIP39 wordlist however, you would have used “addi” for “addict”. Meticulously https://nowbookmarks.com/story19505585/the-2-minute-rule-for-bip39-all-words