For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “increase”, there aren't any extra letters Later on and therefore has to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would've employed “addi” for https://bookmarks-hit.com/story19677347/invalid-bip39-phrase-things-to-know-before-you-buy