For words that only have three letters, there isn't a 4th letter. One example is with the word “add”, there won't be any much more letters afterwards and so should be the word "include". The word “addict” is within the BIP39 wordlist but you would have used “addi” for “addict”. https://ledbookmark.com/story4502000/bip39-seed-phrase-mnemonics-generator-an-overview