For words that only have three letters, there is not any 4th letter. By way of example with the word “increase”, there are no extra letters Later on and so should be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. https://bookmarkusers.com/story18673545/examine-this-report-on-bip39-mnemonic-phrase-tool