For words that only have three letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any extra letters Later on and thus need to be the word "increase". The word “addict” is from the BIP39 wordlist however you might have utilized “addi” for “addict”. https://socialexpresions.com/story5136451/5-simple-techniques-for-bip39-seed-phrase