For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there are no more letters afterwards and thus must be the word "add". The word “addict” is within the BIP39 wordlist however you would've used “addi” for “addict”. In this https://advicebookmarks.com/story26429627/examine-this-report-on-bip39-mnemonic-phrase-tool