For words that have only three letters, there is absolutely no 4th letter. As an example with the word “include”, there isn't any far more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however you would have made use of “addi” https://telebookmarks.com/story9085956/examine-this-report-on-bip39-mnemonic-phrase-tool