For words that only have three letters, there isn't a 4th letter. One example is with the word “include”, there aren't any additional letters afterwards and so must be the word "add". The word “addict” is during the BIP39 wordlist however, you would have used “addi” for “addict”. bip39-phrase.com is https://thesocialdelight.com/story4316155/examine-this-report-on-bip39-mnemonic-phrase-tool