For words that only have three letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any far more letters Later on and therefore must be the word "include". The word “addict” is during the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://thebookpage.com/story4247684/little-known-facts-about-bip39-seed-phrase-list