For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there won't be any a lot more letters afterwards and so should be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://ariabookmarks.com/story4490178/invalid-bip39-phrase-things-to-know-before-you-buy