For words that have only 3 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 needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have applied https://thebookmarkfree.com/story18984784/invalid-bip39-phrase-things-to-know-before-you-buy