For words that only have three letters, there's no 4th letter. As an example with the word “increase”, there won't be any more letters Later on and so should be the word "increase". The word “addict” is from the BIP39 wordlist however , you would have employed “addi” for “addict”. https://brightbookmarks.com/story19053391/fascination-about-bip39-english-words