For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “add”, there are no more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however , you would have utilised “addi” for “addict”. Mnemonic https://todaybookmarks.com/story19104064/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool