For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there won't be any extra letters afterwards and so needs to be the word "include". The word “addict” is in the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://bookmarkstown.com/story19126085/examine-this-report-on-bip39-mnemonic-phrase-tool