For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there are no more letters afterwards and so must be the word "add". The word “addict” is while in the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://bookmarketmaven.com/story19323404/examine-this-report-on-bip39-mnemonic-phrase-tool