For words that have only three letters, there is absolutely no 4th letter. As an example with the word “add”, there won't be any much more letters Later on and thus should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have made https://1001bookmarks.com/story18732818/invalid-bip39-phrase-things-to-know-before-you-buy