For words that only have 3 letters, there isn't any 4th letter. One example is with the word “add”, there won't be any additional letters Later on and therefore should be the word "include". The word “addict” is from the BIP39 wordlist however, you would've applied “addi” for “addict”. Mnemonic https://bookmarkcork.com/story20105503/examine-this-report-on-bip39-mnemonic-phrase-tool